revamp test_refl_angular.py to validate results against theory rather than hard-coded values

[oskooi]

Currently, test_refl_angular.py involves computing the reflectance of a 1d interface for the S polarization at normal incidence and validating the results against hard-coded values. The filename is therefore a misnomer since the test does not actually involve a planewave at oblique incidence. This PR modifies this test to validate the results using the Fresnel equations for two angles of incidence (0° and 20.6°). This makes for a more rigorous and complete test.

This test will eventually be expanded to include a validation of the phase of a total internal reflected mode obtained using mode decomposition via the kz_2d feature as described by @stevengj in #1968 (comment). (The Fresnel equations for the phase are provided in slide 20 of these notes.)

Also included in this PR are small fixes/tweaks to the related Tutorial/Angular Reflectance Spectrum of a Planar Interface.

[oskooi]

Looks like there is an unrelated failure in test_mpb.py involving MPB's output_efield_z run function which involves writing an HDF5 file to disk. The cause of the failure is likely that h5py is trying to write to a file that already exists. This particular failure has been happening recently on the CI virtual machines and should be fixed in a separate PR. Simply restarting the CI fixed it in this PR.

======================================================================
ERROR: test_line_defect (__main__.TestModeSolver)
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Traceback (most recent call last):
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/../../../python/tests/test_mpb.py", line 815, in test_line_defect
    ms.run_tm(mpb.output_at_kpoint(k_points[len(k_points) // 2]),
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/meep/mpb/solver.py", line 970, in run_zodd
    self.run_parity(mp.ODD_Z, True, *band_functions)
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/meep/mpb/solver.py", line 942, in run_parity
    f(self, band)
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/meep/mpb/solver.py", line 1221, in output_efield_z
    ms.output_field_z()
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/meep/mpb/solver.py", line 604, in output_field_z
    self.output_field_to_file(2, self.get_filename_prefix())
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/meep/mpb/solver.py", line 623, in output_field_to_file
    self._output_vector_field(curfield_type, fname_prefix, output_k, component)
  File "/home/runner/work/meep/meep/build/meep-1.22.0/_build/sub/python/meep/mpb/solver.py", line 682, in _output_vector_field
    with h5py.File(fname, 'w') as f:
  File "/opt/hostedtoolcache/Python/3.10.2/x64/lib/python3.10/site-packages/h5py/_hl/files.py", line 507, in __init__
    fid = make_fid(name, mode, userblock_size, fapl, fcpl, swmr=swmr)
  File "/opt/hostedtoolcache/Python/3.10.2/x64/lib/python3.10/site-packages/h5py/_hl/files.py", line 226, in make_fid
    fid = h5f.create(name, h5f.ACC_TRUNC, fapl=fapl, fcpl=fcpl)
  File "h5py/_objects.pyx", line 54, in h5py._objects.with_phil.wrapper
  File "h5py/_objects.pyx", line 55, in h5py._objects.with_phil.wrapper
  File "h5py/h5f.pyx", line 126, in h5py.h5f.create
BlockingIOError: [Errno 11] Unable to create file (unable to lock file, errno = 11, error message = 'Resource temporarily unavailable')

----------------------------------------------------------------------
Ran 47 tests in 260.248s

FAILED (errors=2)

[stevengj]

h5py.File(fname, 'w') should overwrite any existing files.

[stevengj]

We could run into trouble if multiple tests are run in parallel and are writing to files with the same names — we should make sure that the tests use a unique filename prefix for each test (e.g. based on the name of the test file).

[oskooi]

This is good to go.