Add assert_array_shape calls to method code

[tomwhite]

The assert_array_shape call is used a lot in REGENIE, and serves both as a runtime check and as documentation for the reader. We should sprinkle it liberally through other functions.

[ravwojdyla]

Quoting from https://github.com/pystatgen/sgkit/pull/314#discussion_r502548783:

I meant to ask this question many times actually, so we use these asserts pretty generously, which I like, but I wonder if we should ever worry about those assert statements getting optimised out?

[tomwhite]

Good point. I think we should perhaps document it, but I don't think we should come up with a new assertion mechanism.

[ravwojdyla]

@tomwhite yea, I suppose it could be part of this issue to add documentation.