AZFP: add missing mandatory Beam_group1 variables

[emiliom]

Addresses #1100

• beam_type
• transmit_type
• beam_direction_x/y/z

[codecov-commenter]

Codecov Report

Merging #1102 (ec6e95e) into dev (4250acf) will decrease coverage by 0.50%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1102      +/-   ##
==========================================
- Coverage   78.15%   77.65%   -0.50%     
==========================================
  Files          65       18      -47     
  Lines        6234     2927    -3307     
==========================================
- Hits         4872     2273    -2599     
+ Misses       1362      654     -708     

Flag	Coverage Δ
unittests	77.65% <ø> (-0.50%)	⬇️

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Files Changed	Coverage Δ
echopype/convert/set_groups_azfp.py	97.61% <ø> (ø)

... and 49 files with indirect coverage changes

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[emiliom]

Note to self: When addressing beam_direction_x/y/z, it may be helpful to refer back to the AZFP Platform group overhaul PR's merged in the last couple of months, #1058 and #1061

[emiliom]

Added beam_direction_x/y/z variables, set to nan. That is, nothing is being read from AZFP data. The variables are created because they're mandatory (M) in the convention.

[leewujung]

I've been mulling over about this. It is true that CW is Continuous Wave, but it is an approximation. The actual transmit signal is gated sine wave. I wonder if this would be misleading. This is something defined in the convention, but do you think we can add something to make it "Continuous Wave (gated sine wave)"?

[leewujung]

Looks good! I only have a small comment on one of the flag_meanings.

[emiliom]

We've discussed the flag_meanings comment. We decided to stick with "CW", but a new PR will have a more fleshed out flag_meanings entry that better addresses the concern.