Rename fs_receiver in Vendor-specific group

[leewujung]

This PR rename the data variable fs_receiver in the Vendor-specific group to receiver_sampling_frequency. This should have been changed around the time of #1019, but somehow didn't get fixed.

[codecov-commenter]

Codecov Report

Merging #1104 (0280f8d) into dev (13be688) will decrease coverage by 12.11%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              dev    #1104       +/-   ##
===========================================
- Coverage   78.12%   66.01%   -12.11%     
===========================================
  Files          65       30       -35     
  Lines        6227     3911     -2316     
===========================================
- Hits         4865     2582     -2283     
+ Misses       1362     1329       -33     

Flag	Coverage Δ
unittests	66.01% <100.00%> (-12.11%)	⬇️

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Files Changed	Coverage Δ
echopype/calibrate/cal_params.py	91.79% <100.00%> (ø)
echopype/convert/set_groups_ek80.py	82.52% <100.00%> (-14.50%)	⬇️

... and 53 files with indirect coverage changes

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[emiliom]

Nice! And BTW I can see that receiver_sampling_frequency occurs in many places in the code, whereas fs_receiver only occurred here.