prevented mutation of param input and replaced formula for arctanh equivalent
prevented mutation of param input and replaced formula for arctanh equivalent

jseabold/statsmodels,jseabold/statsmodels,josef-pkt/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,jseabold/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,jseabold/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,jseabold/statsmodels,bashtage/statsmodels,statsmodels/statsmodels