Input checking needed for all user-facing methods

[ericbair-sciome]

I have added code to return errors and exit gracefully for all of the user-facing PrestoGP class methods. However, I have also created and exported multivariate versions of several utility functions in GPvecchia (e.g., vecchia_Mspecify, vecchia_Mlikelihood, vecchia_Mprediction, etc.). At the moment, the input checking for these functions ranges between inadequate and non-existent. That should be fixed at some point.