write output files from Python tests to temporary system directory rather than source subdirectory

[oskooi]

Several of the Python tests create output files due to e.g. output_epsilon, save_flux, Animate2D, etc. which are placed in the python/tests subdirectory during make check. As a security measure, some operating systems prevent the test suite from writing any kind of file to disk in non-temporary system directories. This PR modifies the output directory of any test which creates a file to a temporary directory determined by the OS at runtime via tempfile.mkdtemp.

Additionally, all instances of os.path.realpath are replaced with os.path.abspath since on certain systems expanding ".." using realpath is not allowed.

TODOs:

1. since MPB does not contain the equivalent of Meep's use_output_directory parameter, the only test that is left out is python/tests/mpb.cpp.

2. modify the C++ tests in tests which also generate output files to write them to a temporary directory.

note: the tests on Travis are failing due to the following error:

Traceback (most recent call last):
5280  File "../../../python/tests/holey_wvg_cavity.py", line 147, in <module>
5281    temp_dir = mp.comm.bcast(temp_dir, root=0)
5282AttributeError: module 'meep' has no attribute 'comm'

[stevengj]

Would be nice to make this easier to access from C++ and reduce amount of repeated code.

Maybe just update the make_output_directory function in C++ to add a no-argument version that creates a temporary directory?

[stevengj]

Note that the analogous function from C is mkdtemp

[stevengj]

Does mp.make_output_directory('foo', 'bar') work now from Python? Just wondering if the typemaps work here.

[oskooi]

Following #1122, Python's tempfile.mkdtemp has been replaced with meep.make_output_directory. The C++ tests will be updated in a separate PR.