Mask support #131
Description
Currently masking is not fully implemented, implement this.
Decisions will need to be made about the strategy to combine masks when present for multiple datasets, depending on the operation the choice might be different.
For example, for basic arithmetic operations, we probably want to and them. For something like means, we might want to or them. All of this can be tracked through the quantity objects, and the operations that apply to masks need not be in any way identical to those on the data.