Warning for small smoothness values

[mmaelicke]

An issue came up during #88: skgstat can fit very small smoothness values for a Matern model, which is not really useful, as explained here: https://github.com/mmaelicke/scikit-gstat/pull/88#issuecomment-821026903

So, this issue is a reminder to tweak Variogram.fit to use a lower limit for smoothness values. Should be a one-liner.