patch method iselect

[d-chambers]

Description

This PR adds a patch method called iselect which is similar to select except it uses indices rather than values for the selection.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

Merging #198 (f815488) into master (9134243) will decrease coverage by 0.96%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   98.65%   97.69%   -0.96%     
==========================================
  Files          74       74              
  Lines        5557     5592      +35     
==========================================
- Hits         5482     5463      -19     
- Misses         75      129      +54     

Flag	Coverage Δ
unittests	97.69% <100.00%> (-0.96%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dascore/core/coordmanager.py	100.00% <100.00%> (ø)
dascore/core/coords.py	100.00% <100.00%> (ø)
dascore/core/patch.py	85.35% <100.00%> (-14.65%)	⬇️
dascore/proc/__init__.py	100.00% <100.00%> (ø)
dascore/proc/select.py	95.00% <100.00%> (-5.00%)	⬇️

... and 5 files with indirect coverage changes