Add capability to created wanted variables list with an explicit list of paths

[andypbarrett]

This pull request adds the capability to the Variables.append method to create wanted variables using a list of explicit paths.

• a path_list keyword argument was added to append.
• parse_var_list and set are used to generate unique var_list, beam_list, keyword_list variables. These are then used as they had been in the original version of append.
• the docstring for append was updated to describe the new keyword and give a usage example.
• keyword checking is added to ensure path_list but not var_list, beam_list, or keyword_list are set. A ValueError is raised if this check fails.
• The check that at least one combination of keywords is set was updated to include path_list and is modified to use if instead of assert. This allows a more informative Exception to be raised than an AssertionError

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[JessicaS11]

Thanks for the great PR, @andypbarrett! My comments are mostly typos, and hopefully a fix to get the tests via Travis to pass.

[andypbarrett]

Thanks @JessicaS11 and @rwegener2 . I plan on fixing these on Thursday 15 Feb.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 10.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 66.02%. Comparing base (de13727) to head (e294e31). Report is 1 commits behind head on development.

Files	Patch %	Lines
icepyx/core/variables.py	10.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #508 +/- ## =============================================== - Coverage 66.19% 66.02% -0.18% =============================================== Files 36 36 Lines 3065 3073 +8 Branches 541 544 +3 =============================================== Hits 2029 2029 - Misses 945 953 +8 Partials 91 91 ```

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