prestogp_fit does not allow one to specify a distance function

[ericbair-sciome]

vecchia_Mspecify allows one to define a custom distance function. However, prestogp_fit does not allow one to specify a distance function and defaults to Euclidean distance. We need to add a parameter to prestogp_fit to specify a distance function.

[kyle-messier]

@ericbair-sciome @sciome-bot I remember this being particularly important if we need to allow distance in miles vs kilometers. Moving to v1.0 unless that point is obsolete.