sensitivity kernel (banana-doughnut) adjoint source

[bch0w]

Include sensitivity kernels as a valid adjoint source type, which is simply feeding a manipulated form of the synthetic seismograms back as the adjoint source. This has already been coded up here:

https://github.com/adjtomo/pyatoa/blob/3690bc8bc59c76dcfdf31885259dc55a4efd681e/pyatoa/utils/adjoint.py#L9-L63

But will need to be written in PyAdjoint formatting