ndarray/xarray based Composite Key Checker

[DocOtak]

As part of the "fast" reader work, functions/methods are needed to directly check an xr.Dataset or potentially ndarrays for valid exchange coordinates. This include:

• The variables of expocode, station, cast, sample are compatible numpy (or xarray) broadcasting.
• After broadcasting, the elementwise concatenation of values of the above variables is either unique or empty.
• Data (after broadcasting) exists only if and only if the above elementwise is not empty.

This will allow us to check arbitrary input netcdf files for CCHDO flavored CF/netCDF validity not just the ones created by this software.

[DocOtak]

This was not closed in #27 because, while that checks the keys while converting, we still need a checker function that tells us if the xr.Dataset is correct outside of the exchange conversion context.