Add the non-conservative form of the SGS stress divergence

[ia267]

Implement the non-conservative form (iconserv = 0 in Incompcat3d - this is the default). It applies the product rule analytically first then discretise, where we can move the non-constant nu_t out of the the derivative (then we can use der2nd).

At the moment I have only implemented the conservative form because the atmospheric boundary layer wall model needs it. I'll raise the non-conservative form as a separate issue.

Originally posted by @ia267 in #338 (comment)