Enable missing_value to override default default_value for fields

[mgduda]

Following a discussion on 4 May 2020, there is interest in having the the default value for a field take on the specified missing_value if no explicit default_value is given in the registry entry for a field.

The motivation for this change is that fields that do contain missing values would otherwise need to be explicitly initialized by a core to the missing value; initializing a field to its registry-specified missing_value would remove this burden from the core developer.

[xylar]

@mgduda, yep, that looks like what we discussed. Thanks for being willing to implement this!

[MiCurry]

And if both missing_value and default_value are present, then the field's default value should be set to default_value? i.e. the current behavior?

[xylar]

@MiCurry, my hope is that missing_value and default_value wouldn't be specified for the same field because that would be confusing.

@mgduda, is the difference in behavior between missing_value and default_value whether this attribute ends up in the NetCDF file or not (missing_value: yes; default_value: no)?

[mgduda]

@xylar Aside from differences in how these two attributes are used to set the initial value of a field after it has been allocated, that's correct: the missing_value attribute is added as a netCDF variable attribute, while the default_value attribute is not.