make np.datetime64 units "ns" explicitly

[JessicaS11]

makes explicit (rather than relying on default) the numpy datetime64 units as "ns" (Xarray default changed and was guessing "us" as more appropriate units instead, causing the _make_np_datetime docstring test (read.py) to fail).

[github-actions]

👈 Launch a binder notebook on this branch for commit 84c4258

I will automatically update this comment whenever this PR is modified

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.77%. Comparing base (2f1875a) to head (84c4258).
Report is 15 commits behind head on development.

Files with missing lines	Patch %	Lines
icepyx/core/read.py	50.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (2f1875a) and HEAD (84c4258). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (2f1875a)	HEAD (84c4258)
	5	2

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #653      +/-   ##
===============================================
- Coverage        71.78%   66.77%   -5.02%     
===============================================
  Files               38       36       -2     
  Lines             3137     3079      -58     
  Branches           426      426              
===============================================
- Hits              2252     2056     -196     
- Misses             775      936     +161     
+ Partials           110       87      -23     

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