make sub() behave like ref()

[carlobaldassi]

i.e. drop trailing singleton dimensions, as discussed here.

Example:

julia> a = rand(2, 3, 2)
2x3x2 Float64 Array:
[:, :, 1] =
 0.121652  0.232535  0.0812166
 0.347117  0.224052  0.885407 

[:, :, 2] =
 0.626699  0.342665  0.163809
 0.253675  0.659507  0.922081

julia> sub(a, 1:2, 2, 1)
2-element SubArray of 2x3x2 Float64 Array:
 0.232535
 0.224052

I think it's correct, but since it's not a trivial change, I'll leave it here for review/further discussion instead of pushing it directly.
Also, there's still a difference in behaviour, since sub still only allows either 1 index or the same number of indices as the parent array, but I think it was decided it's ref which has to change in that case.

[pao]

This looks like an accidental edit.

[carlobaldassi]

thanks for the catch!

[JeffBezanson]

Looks good, but we need to abstract out the function that determines shape from indexes. When we do that this will have to be edited again.

[carlobaldassi]

So if there are no objections I could merge this now to get the functionality, and we'll refactor the code when the abstraction for determining shape from indices is provided.

[StefanKarpinski]

Go for it.