More fixes

[gboehl]

1. The 'solver' argument for solved_blocks was being ignored. This is because the check was if 'solver' not in kwargs, but solver was defined by default, but an empty sting. The string evaluates as False, but solver in kwargs as True because the key is present.

2. Two bugs in function parsing:

   1. The last occurence of the return-statement was evaluated instead of the first (as common in python)
   2. return statements in commented lines are evaluated anyways. This one was nasty.

3. Allow *.appy() to take endogenou variables as kwargs. This fix worked for me, but is probably incomplete given that apply() is a method of other classes as well.

Example for (2.):

@simple
def func0(input):
   stuff = input
   return stuff

# def func1(input):
#  return thing

would evaluate as

@simple
def func0(input):
   stuff = input
   return thing