move lambertw to singlediode-methods module

[mikofski]

Problem
some methods are in singlediode_methods while others are in pvsystem, makes sense for them to be together. also according to this comment

this is a lot of lines to read within an if/else...

Solution
move lambertw to singlediode_methods

alternatives
do nothing

Additional context
see #408 , #409 , & #410

[mikofski]

@cwhanse and @wholmgren - maybe someone already asked this, but why didn't we just call this module "singlediode" why append _methods? seems like a redundant, no? Should we change it before v-0.6.0?

[wholmgren]

@mikofski I don't recall a discussion of singlediode.py vs. singlediode_methods.py. I have a slight preference for singlediode.py.