convenience functions + add_param takes std not var
convenience functions + add_param takes std not var

convenience:
- directly access parameter names with attribute 'names'
- sample from parameter distribution
- sample from transformed parameter distribution

add_param now takes the standard deviation as input, not the variance, because
this is easier to compare to the mean

sbitzer/pyEPABC,sbitzer/pyEPABC,sbitzer/pyEPABC