Remove redundant ping_time dimension

[leewujung]

Similar to #978, the convention requires the beam dimension for many variables, but for the echosounder models we support many of these variables do not change over ping_time during a deployment or survey, since they are properties of the transducers. This dimension was artificially created in the current code.

Since ping_time is a major dimension that data is being concatenated on, this repetition is costly when data is loaded into memory (though it compresses well). I think we should consider removing this redundant dimension.

@emiliom : tagging you so that it goes with work in #1056.

[leewujung]

Addressed in #1083.