Add regularised visibility calculation

[samaloney]

1. Visibilities with an observer coordinate frame (d_sun)
2. Reorder vis so for each (u,v) as a function of energy
3. Rescale spectra by fitting a single, broken or sum power laws pick by lowest chi-square (divide by fit to have similar magnitude number to make inversion possible/faster)
4. Reject low energy bins if error too large (magic number)
5. Need the SRM for vis energies (attenuator)
6. get bremsstrahlung cross section
7. estimate electron flux simple multiplication -> 1st estimate of electron vis
8. SVD on matrix SRM * cross section * energy (cm^2 keV-1) [ for photon could do on SRM * energy (cm^2 keV-1)]
9. regaulisation using SVD, with both discrepancy and 3-sigman -> reg electron vis
10. confidence strip - perturb the count spectra repeat above to obtain an estimate of confidence