add missing_value attribute

[gdkrmr]

YAXArrays.jl needs a missing_value attribute to properly support the Missing type.

The CFConventions seem to mention that missing_value is only valid for Integers (I haven't confirmed this yet).

[gdkrmr]

@davemlz do you have a reference, I cannot find it neither in [1] nor [2].

[1] https://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#missing-data

[2] https://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#ch9-missing-data

[gdkrmr]

If only one missing value is needed for a variable then we recommend that this value be specified using the _FillValue attribute. [...]

The scalar attribute with the name _FillValue and of the same type as its variable is recognized by the netCDF library as the value used to pre-fill disk space allocated to the variable. This value is considered to be a special value that indicates undefined or missing data, and is returned when reading values that were not written.

I would like to hear what @meggart has to say about this. Can we invite him this repository?

Also: I cannot find anything in the CFConventions that discourages missing_value for floating point numbers.