Covariance matrix check

[Ian Culverwell]

At present there is no check that an injested covariance matrix has the right size for the problem in hand. Because of the way covariance matrices are stored in ROPP (in a "triangular" format), a matrix appropriate to a bigger problem can be read in OK, without crashing and with 1s in the right place (think about it). So it would be subtly wrong - the worst kind of error.

At least one user has fallen over with this. So we should put a check in to stop it happening.