Added class ConstantFunction
Added class ConstantFunction

Trivial addition to HARKinterpolation.py: ConstantFunction.  For
representing functions that return the same value no matter what the
input (or how many inputs).  Yes, this is actually useful.

econ-ark/HARK,mnwhite/HARK,econ-ark/HARK,bolmez/Class-HARK