Rename SelectTransform by Mask

[theogf]

Just realised that Mask was the most used name for this.

[willtebbutt]

I have no strong feelings either way.

[theogf]

I will just do the best of both world and add an alias for it.

[theogf]

Should we still do this? I feel Mask could actually replace completely SelectTransform.

[willtebbutt]

Thinking about this again, I think that we should be careful with the semantics, and I'm not entirely convinced that I'm as neutral as I was before.

In particular, I would have imagined that the functionality of Mask would be to remove the dimensions where the boolean-valued vector is 1 while retaining the dimensions where the vector is 0 -- a Mask says where to "mask out" the inputs. Conversely, for Select I imagine the exact opposite behaviour. Does this make sense? I think that the term mask gets used quite a lot in common parlance within various fields with the same semantics as Select, but it feels a bit weird when you think about it...

I think I generally prefer to say which inputs to keep, rather than which to throw away, so I wonder whether sticking with Select would make more sense?

I could probably be convinced either way.