properly apply all dimensions to deeply nested variables

[JessicaS11]

In debugging #620, it turned out that variables with a fourth layer of nesting (e.g. ATL06's gt2r/land_ice_segments/geophysical/bsnow_h) were being added to the Xarray Dataset but missing some of their dimensions. This PR addresses that by adding any dimension of size 1 in the subgroup-read-in step to that variable before merging it with the multi-granule dataset. This allows per-granule datasets to be properly merged automatically (rather than icepyx returning a list of per-granule datasets).

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.77%. Comparing base (50af59d) to head (4514fd2).
Report is 1 commits behind head on development.

Files with missing lines	Patch %	Lines
icepyx/core/read.py	0.00%	1 Missing ⚠️

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HEAD has 23 uploads less than BASE

Flag	BASE (50af59d)	HEAD (4514fd2)
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Additional details and impacted files

@@               Coverage Diff               @@
##           development     #623      +/-   ##
===============================================
- Coverage        71.81%   66.77%   -5.04%     
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  Files               38       36       -2     
  Lines             3136     3079      -57     
  Branches           426      426              
===============================================
- Hits              2252     2056     -196     
- Misses             774      936     +162     
+ Partials           110       87      -23     

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