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:

• [x] This PR addresses an already opened issue (for bug fixes / features)

  • This PR fixes a bug in issue #288 ; subset_shape(...) would return all points in the bounding box without filtering the points inside the polygon.

• [x] Tests for the changes have been added (for bug fixes / features)

• [ ] Documentation has been added / updated (for bug fixes / features)

• [x] HISTORY.rst has been updated (with summary of main changes)

• [x] 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

[Zeitsperre]

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

[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.