Allow one to query the size of an existing dimension

[d7919]

Allows one to query the size on an existing dimension. For symmetry we could perhaps also allow the values based overload to return the actual values (as this doesn't have a dim_size argument) or we could add a dim_size argument to these overloads (requiring a check that dim_size and values aren't set, or at least if both are set then are consistent).

[d7919]

This would require neasyf_dim to only ever pass a variable to dim_size rather than the more commonly desirable integer literal so this is not the correct approach for such functionality.