transients: fix window=none case and allow custom dt0, dtau
This fixes the windowRange struct for type=none, in accordance with XLALComputeTransientFstatMap. And since that meant I needed to touch the if-else construct for default t0, tau spacings anyway, I chose this opportunity to make them user-configurable too.
It seems somewhat messy and contrary to the classes idea to keep adding extra transient parameters to the core ComputeFstat class. But we talked in the past about a "hybrid" MCMC search that only samples in Doppler params, with the transient params marginalised over a grid, and I think to keep that option open, it's probably best to continue in this way.