resolve environment variables in Path traitlets

[kosack]

This small change allows one to use env vars in config files whenever a Path is requested. On the command-line, your shell already expands them, so this is mostly useful for configuration files.

e.g:

"DataWriter": {
    "output_path": "$HOME/Data/events.DL1.h5"
}

>>> print(d.output_path)

PosixPath('/Users/kkosack/Data/events.DL1.h5')

This is useful, for example, if you want to define your configuration files in relationship to some base directory.

[codecov]

Codecov Report

Merging #1855 (7042af4) into master (d7f8a58) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1855   +/-   ##
=======================================
  Coverage   92.22%   92.23%           
=======================================
  Files         190      190           
  Lines       15196    15211   +15     
=======================================
+ Hits        14015    14030   +15     
  Misses       1181     1181           

Impacted Files	Coverage Δ
ctapipe/core/tests/test_traits.py	100.00% <100.00%> (ø)
ctapipe/core/traits.py	95.14% <100.00%> (+0.01%)	⬆️
ctapipe/coordinates/tests/test_coordinates.py	100.00% <0.00%> (ø)

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