Document n_chap variable

[Ian Culverwell]

ROPP8.0 beta reviewer Ben Ruston (NRL) comments:

The %n_chap variable in the State1dFM structure is not included in
Table 4.2 of the romsaf_ropp_ug_1dvar.pdf.

This is a deliberate mistake. state%n_chap is currently hardwired to 1. We don't want people to think they can add as many CLs as they like. Extending the scheme to allow more than one Chapman layer would involve a lot of 'administration' (increasing size of augmented state vector, rewriting the rescaling of the ionospheric components of the covariance matrices, etc). This ticket is therefore being opened as a reminder to update the documentation in the unlikely event that this generalisation is undertaken.