Update a couple of "extra" EK60 Beam_group1 variables

[emiliom]

Follows from #951 (comment), but for EK60. Complements PR #1094.

In Beam_group1:

• Drop count variable
• Rename offset to range_sample_offset
• Cast some variables to integer types: range_sample_offset to dtype=int, transmit_mode to dtype=np.byte (analogous to channel_mode in EK80) and data_type to dtype=np.byte.
• Improve attributes for range_sample_offset, transmit_mode, and data_type; partly for consistency with EK80

NOTES:

• The casting to integer types is currently not working! The EchoData DataArrays end up being float types. I'm looking into it.
• transmit_mode seems to be identical to channel_mode in EK80. @leewujung can you confirm? If they are, let's use the same variable name (channel_mode? Or change EK80 to transmit_mode?) and attribute values.

[emiliom]

• Renamed transmit_mode to channel_mode and harmonized its attributes with the attributes from ek80 channel_mode
• Finalized the specific int types used
• Ensured that int casting works as intended by adding the following to EXPECTED_VAR_DTYPE in utils/coding.py:

      "channel_mode": np.byte,
      "range_sample_offset": np.int32,

[codecov-commenter]

Codecov Report

Merging #1099 (c6c7a5f) into dev (6fc5a1d) will decrease coverage by 3.07%.
Report is 20 commits behind head on dev.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1099      +/-   ##
==========================================
- Coverage   78.12%   75.06%   -3.07%     
==========================================
  Files          65       23      -42     
  Lines        6227     3393    -2834     
==========================================
- Hits         4865     2547    -2318     
+ Misses       1362      846     -516     

Flag	Coverage Δ
unittests	75.06% <ø> (-3.07%)	⬇️

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Files Changed	Coverage Δ
echopype/convert/set_groups_ek60.py	92.74% <ø> (ø)
echopype/utils/coding.py	29.88% <ø> (-59.78%)	⬇️

... and 53 files with indirect coverage changes

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[leewujung]

This looks great. Thank you!

[emiliom]

Great, thank you!