define the "inner_mask" for locstream case

[vindelico]

Define the inner_mask in subset_shape() for the locstream case.
subset_shape() would return all lat/lon inside inner_mask which is equivalent to the bounding box of the polygon, not the area inside the polygon.

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes a bug in issue Add support for locstream lon/lat in bbox and shape subsetting #288 ; subset_shape(...) would return all points in the bounding box without filtering the points inside the polygon.
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
• I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?:
  The inner_mask in subset_shape() is defined for the locstream case.

• Does this PR introduce a breaking change?:
  No.

• Other information:

[coveralls]

Pull Request Test Coverage Report for Build 5662181592

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 78.516%

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Totals
Change from base Build 5579360884:	0.06%
Covered Lines:	1111
Relevant Lines:	1415

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💛 - Coveralls

[aulemahal]

Could you add a test for this? I think test_locstream (

clisops/tests/core/test_subset.py

Line 877 in c49fcc3

def test_locstream(self):

) could be modified so that the test polygon is not a square.

[Zeitsperre]

@vindelico Do you have any free time to work on/finalize this PR? I have use cases that could benefit from your changes!

[aulemahal]

The changed test seems to answer my initial suggestion. Looks good!

[vindelico]

@vindelico Do you have any free time to work on/finalize this PR? I have use cases that could benefit from your changes!

Can you check HISTORY.rst and AUTHORS.rst, plz? - otherwise I don't have additional changes.