sigvar needs general way of handling masked data

[tgperring]

When any object that has masked data has binary operations implemented on it, the current method for handling it has to be implemented in a bespoke fashion. However, sigvar can in principle support a consistent way of handling this situation. Sigvar lies at the root of unary and binary operations, and in the new Matlab classdef implementation it has a property 'msk' which can be used for this purpose which is currently not used.

It will require some additional required methods for classes that use sigvar to extract the mask, as well as modification to the code of sigvar to use 'msk' internally. Not high priority.

[tgperring]

Addendum: The one potential priority is to reolve the case of binary operations for dnd objects in Horace: currently if there are empty bins (i.e. npix=0 for one or more bins) in both w1 and w2 in an operation like w1+w2, then this is not handled correctly. The fixup in Horace that existed prior to the the refactoring of class sigvar has been removed in anticipation of this ticket implemented; there was no uinit test that caught this situation.