Add missing basis functions

[danielward27]

I believe currently missing are:

1. smooth.construct.ad.smooth.spec - Adaptive smooths
2. smooth.construct.gp.smooth.spec - Gaussian process smooths
3. smooth.construct.so.smooth.spec - Soap film smooths
4. smooth.construct.sz.smooth.spec - Sum to zero smooth interactions

They should be added to basis_functions.py, and test cases probably added to spline_test_cases in gam_test_cases.py. spline_test_cases should be renamed to be something like basis_function_test_cases and ideally include a case for each basis at least.