Overhaul AZFP Environnment group

[emiliom]

Currently the AZFP Environment group is created with temperature and pressure variables, even if the variables have no valid data. Since pressure data are almost never present, and these variables are not defined by SONAR-netCDF4, always creating these variables is unnecessary and wasteful.

Also, the mandatory variables absorption_indicative and sound_speed_indicative are not created.

• [ ] Always create absorption_indicative and sound_speed_indicative variables (float type). In SONAR-netCDF4 they have a frequency dimension; here, they will have a channel dimension. At this time I don't know where the values would come from, so set to them to np.nan
• [ ] Create an associated frequency_nominal(channel) variable
• [ ] temperature will be created only if self.parser_obj.unpacked_data["temperature"] contains valid data
• [ ] pressure will be created only if self.parser_obj.unpacked_data["pressure"] contains valid data

The attributes for absorption_indicative and sound_speed_indicative are taken from the convention:

# absorption_indicative
{
    "long_name": "Indicative acoustic absorption",
    "units": "dB/m",
    "valid_min": 0.0,
}
# sound_speed_indicative
{
    "long_name": "Indicative sound speed",
    "standard_name": "speed_of_sound_in_sea_water",
    "units": "m/s",
    "valid_min": 0.0,
}

Note that if neither temperature nor pressure contain valid data, the variables won't be created, and the time dimension and coordinate variable should not be created either.

[leewujung]

This can be closed now since #1226 is merged.