Add optional "long_name" to "axes"

[thewtex]

The "axes" metadata currently contains a "name" that is typically { "x", "y", "z", "c", "t' }.

We could also add support for an optional "long_name" to the "axes" metadata. This would be a semantic name that could be standardized across domains, e.g. "anterior-posterior", "width".

While standard, canonical values could help interpretation, this adds more flexibility and the possibility for additional, sometimes crucial metadata. For visualization, this would be the name of the axes label.

This also improves xarray, NetCDF compatibility, so there is a corresponding long_name and units:

https://docs.unidata.ucar.edu/netcdf-c/current/attribute_conventions.html

[thewtex]

xref #48

[bogovicj]

@thewtex

I like it.
No additional constraints on name right? and no constraints on long_name either right?

Also, the spec likes camel case, so is longName okay by you? or description?

[thewtex]

No additional constraints on name right? and no constraints on long_name either right?

Yes.

Also, the spec likes camel case, so is longName okay by you? or description?

longName is fine by me. description is another possibility, but I it would be nice to keep the association with the netCDF counterpart if possible.

[joshmoore]

"display" or "displayName" come to mind as well, but I'll give the attribute conventions doc a good reading.