Kalasiris

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Added
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* New kalasiris.set_persistent_preferences() functionality so that a library user can set
a persistent preferences path rather than having to enter it with each kalasiris
function run. Addresses Issue 11.
* Can now also pass parameters that do not take an argument to ISIS in a manner similar
to how you'd write them on the command line (e.g. `getkey("-help")`) in addition to
the "old" way (e.g. `getkey("help__")`).

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Changed
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* Testing now defaults to only running on mocks.
* Consolidated boolean that controls testing on "real files" to single place.
* Enabled GitHub Workflows for testing.

Fixed
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* Bug in reserved parameter handling, such that only some reserved parameters
that were given with two trailing underbars (_) would be converted to their
appropriate "-key=value" form for ISIS. This was true for "restore" for example,
which was embarrassingly given in the documentation, but not tested!
* PathSet module documentation was missing the first letters of several words.

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Added
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* Now supports the new ISISDATA environment variable, in addition to ISIS3DATA.

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* Added the ability to pass arguments to ``subprocess.run()`` for each ISIS program.
* The ISIS functions now automatically log with a level of INFO.
* Adopted Conventional Commits pattern (https://www.conventionalcommits.org/en/v1.0.0) for commit messages.
* Adopted Black formatting.
* Minor spelling and formatting improvements.

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* Corrected a readthedocs failure.

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* Added some functions to read table data from cube files.
* Added special pixel values via specialpixels.py (implemented
as namedtuples instead of as dictionaries, as in pysis).
* Enabled Python 3.7 and 3.8 tests for tox.