Obsolete diagnostics are written to the available_diagnostics file

[nichannah]

Obsolete diagnostics get registered and hence turn up in the available_diagnostics file. Is there a particular reason why these are still being registered? Is it just so that the replacement diagnostic can be printed?

If so, I suggest we use FMS:get_diag_field_id() to check whether old diagnostics are in the diag_table. This way a replacement can be suggested without registering the diagnostic.
#147 depends on this, will probably fix it there.

[nichannah]

Oh, I've just read found_in_diagtable(), it seems that this problem of suggesting without registering is solved.

[nichannah]

OK, I was confused. There seems to be only one case where this is happening. Fixed in #147.

[nichannah]

Reopened just to keep track of everything that needs to make it into #147.