fixing python dtype conversion of tuple(str)

If you create a Model with only one parameter that has more than one character, the GP class will raise an error:

`from george.modeling import Model import george import numpy as np

class PolynomialModel(Model): parameter_names = ("amp")

def get_value(self, t):
    t = t.flatten()
    return (t * self.m + self.b +
            self.amp * np.exp(-0.5*(t-self.location)**2*np.exp(-self.log_sigma2)))

model = george.GP(mean=PolynomialModel(amp =-1)) `

This will return

raise ValueError("missing parameter '{0}'".format(k)) ValueError: missing parameter 'a'

However, if the parameter_names entry is only one character (e.g. 'm') it will work as expected. There is a simple solution to this problem: use parameter_names = ("amp", ) to force python to create a tuple (instead of str type)

However, this is not documented and, in my opinion, not the intended behaviour for the Model interface. This PR just checks if the self.parameter_names is a string and, in that case, create a tuple.

dfm / george

fixing python dtype conversion of tuple(str) #147