Add UxDataset.isel() (#1352)

* first rev

* add tests

* implement ds.isel

* add docstring

* move test_isel to core

* remove isel test from test_inheritance in favor of seperate module

* add isel to api

* move tests into test_inheritance, reogranize tests

Author: philipc2

docs/api.rst

@@ -176,6 +176,14 @@ Constructors
    UxDataArray.from_xarray
    UxDataArray.from_healpix
 
+Selection & Indexing
+~~~~~~~~~~~~~~~~~~~~
+
+.. autosummary::
+   :toctree: generated/
+
+   UxDataArray.isel
+
 Grid Accessor
 ~~~~~~~~~~~~~
 
@@ -207,6 +215,14 @@ Grid Accessor
 
    UxDataset.uxgrid
 
+Selection & Indexing
+~~~~~~~~~~~~~~~~~~~~
+
+.. autosummary::
+   :toctree: generated/
+
+   UxDataset.isel
+
 
 Conversion Methods
 ------------------