Add LOS data to MCMC

[elc45]

This PR adds line-of-sight data to the MCMC model in the following way: the config is given a new entry los_file_name pointing to the csv, much like for stations. Once loaded into the model, a new obs dataframe is made, which is a concatenation of the stations and los files. This is then used instead of stations to create the operators. Then, at the end of the forward model, a likelihood is applied to all observations in this dataframe conditional on whether the observation is a station or an LOS point (using an is_los col). If the former, the usual likelihood is applied; if the latter, a modified likelihood which dots the (N,E,U) output with the look vector, then takes a Normal likelihood with sigma ~ HalfNormal(0,1) * los_err. We could also change this to a Student's T if problems arise.