Fix for pp load rule with hybrid model level number

There is a problem with the pp load rule logic for files that use hybrid vertical coords ie. lbvc = 9.  No value is loaded in for the level number in this instance.

I added a new rule to the pp_rules.txt file and generated pp_rules.py with SciTools/iris-code-generators:tools/gen_rules.py. It has changed more than I expected to, but passes the tests. Also, there is currently a commented out section in the pp_rules.txt which says "hybrid pressure needs further definition". I'm guess this was meant to be fixed to sort the problem I've been having.

I'm not sure how/if this new rule should be tested.