Enable evaluation of models at custom times and locations

[smithara]

Use a template vires_fetch_model_async.xml to evaluate models using a list of [lat, lon, rad, time] points as input.

This could put heavy load on the server when the number of points is very large. It could alternatively be done on the client side by fetching model parameters and passing them a local installation of eoxmagmod (or some future equivalent). Ultimately I think both options should be available (with the latter probably not in viresclient) - so that users can easily get model evaluations via vires without the extra software installation and client-side load, but when they need (and/or have available) more performance then it could be possible for them to do it faster on their machine.