Allow for optional kwargs for loading group

[lsetiawan]

Overview

This PR allows for being able to pass in additional xr.open_dataset arguments to open_converted for example adding chunks='auto' will delay the arrays in each group so that one can combine without blowing up memory.

Additionally, there's a fix to the datatype of src_filenames, ensuring that they're always a string.

[codecov-commenter]

Codecov Report

Merging #456 (02f5239) into dev (900f4bc) will decrease coverage by 3.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##              dev     #456      +/-   ##
==========================================
- Coverage   76.77%   73.74%   -3.04%     
==========================================
  Files          38       11      -27     
  Lines        3324      697    -2627     
==========================================
- Hits         2552      514    -2038     
+ Misses        772      183     -589     

Flag	Coverage Δ
unittests	73.74% <83.33%> (-3.04%)	⬇️

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

Impacted Files	Coverage Δ
echopype/echodata/api.py	16.66% <ø> (-66.67%)	⬇️
echopype/calibrate/calibrate_ek.py	89.88% <66.66%> (-0.67%)	⬇️
echopype/echodata/combine.py	60.68% <100.00%> (-10.01%)	⬇️
echopype/echodata/echodata.py	58.82% <100.00%> (-28.87%)	⬇️
echopype/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
echopype/echodata/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
echopype/echodata/convention/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
... and 28 more

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