increase max sampling factor in refinement modules

[mgiulini]

PR #658 added checks on parameters' values. Therefore, the max value of sampling_factor should be higher than one, otherwise it does not make sense to have this parameter. I propose to use 100 as maximum value (high enough to have variability when needed, for example in single structure refinement) and to modify the python code so as to handle possible mistakes when the user has, say, 1k models and sampling_factor = 100 (this should not be allowed).

Default and min values are correct.