Align the attributes of the PModel and SubdailyPModel implementations

[davidorme]

We currently have completely inconsistent naming for the attributes of the PModel and SubdailyPModel. The table below isn't yet complete - I think the thing to do is probably make the __init__ methods logic free and have a complete listing of attributes and docstrings there, that then calls a private _fit_model method to isolate the logic.

See also #385 and #384 which overlap considerably. Probably close those two first - a bit around the houses but reducing the number of things being changed in one PR. This one is a big one.

Use	Current PModel	Current SubdailyPModel
lue	lue	???
iwue	iwue	???
vcmax	vcmax	subdaily_vcmax
vcmax25	vcmax25	subdaily_vcmax25
vcmax25_daily_optimal	NA	vcmax25_opt
vcmax25_daily_realised	NA	vcmax25_real
jmax	jmax	subdaily_jmax
jmax25	Not currently exposed	subdaily_jmax25
jmax25_daily_optimal	NA	jmax25_opt
jmax25_daily_realised	NA	jmax25_real
xi	self.pmodel_acclim.optchi.xi - add a getter?	subdaily_xi
xi_daily_optimal	NA	self.pmodel_acclim.optchi.xi Maybe add a getter?
xi_daily_realised	NA	xi_real
gpp	gpp	gpp
gs	gs	???
ppfd	ppfd	???
fapar	fapar	???
rd	rd	???
Ac	Not currently exposed	subdaily_Ac
Ac	Not currently exposed	subdaily_Aj
J	Not currently exposed	`Not currently exposed