Change shape of returned value for `transpose_matvec` and `sandwich` about tabmat

[tbenthompson]

This seems like a really important design decision that we should consider. I think the default here may not be the right choice. We should consider the influence on how these recursive restrictions occur in SplitMatrix.

[tbenthompson]

The current solution of returning an array of shape (k2,) seems sufficient. Arguments:

• That shape is what quantcore.glm expects.
• The reduced shape gives the user more options. The user can always insert the resulting array into a larger array with zeros.
• The reduced shape is more efficient.

I think these arguments are strong enough that I'm going to close this issue.