Allow for custom misfit functions

[larsgeb]

I want to include a routine on the Python side that allows for custom misfit functions and adjoint sources. This can be easily facilitated by using

psvWave/src/psvWave.cpp

Line 460 in de1b8a9

.def("get_synthetic_data", &fdModelExtended::get_synthetic_data)

to get the forward result and writing a new interface to set the adjoint sources. These should override the fdModel.calculate_l2_misfit and fdModel.calculate_l2_adjoint_sources methods, possibly aliasing that first to something more general, i.e. calculate_misfit.