Move iglo loop into invert_rhs_1 and restructure

Moving the iglo loop into invert_rhs_1 can help make the code clearer and enables future changes to e.g. add openmp, calculating the entire source at once etc.

Restructures the loop body a little to group together related code.

Would be good to also rationalise the logic regarding wfb treatment and other boundary treatments. This may then enable similar changes to those proposed in PR #205