(derived) Setting a filter expression to define the base of an array subvariable

[jamesrkg]

This is related to/clarifies #196 and #215.

We need a way to use a filter expression to define the base for an array subvariable. All rows that do not satisfy the filter expression should be set to missing for that subvariable.

This is related to #215 which requests for the same sort of control but at the variable-level, rather than the subvariable-level.

Example for clarity (not necessarily an API suggestion):

ds[alias][subalias].base_filter = "valid(other_subvariable)"

[xbito]

@jjdelc is that possible in the API?

[jamesrkg]

As with #215 this will actually be addressed by #228, so I'm closing it.