Add parameter checks for parametric distributions

[sallen12]

Second breakdown of #101

• Add parameter checks when calculating the CRPS and log score for parametric distributions. An additional argument "check_pars" is added to the score functions, so that the checks can be removed for more efficient optimisation of the scores as loss functions.

The checks could possibly be written into utility functions, since they are used for both the log score and CRPS - the current code is therefore slightly repetitive. Let me know if you would prefer this.