Returns a Fisher object based on user-input.

def _init_pval_obj(self):
        """Returns a Fisher object based on user-input."""
        if self.pval_fnc_name in self.options.keys():
            try:
                fisher_obj = self.options[self.pval_fnc_name](self.pval_fnc_name, self.log)
            except ImportError:
                print("fisher module not installed.  Falling back on scipy.stats.fisher_exact")
                fisher_obj = self.options['fisher_scipy_stats']('fisher_scipy_stats', self.log)

            return fisher_obj

        raise Exception("PVALUE FUNCTION({FNC}) NOT FOUND".format(FNC=self.pval_fnc_name))