add functions to convert the parameters dictionary to a numpy array and back

[beckermr]

This function is needed to interface with other tools, like emcee etc. Note that in many versions of python, dictionaries don't have any ordering promises on their keys. Thus this function needs to do something like sort the keys of the dict, and return the values in the key sorted order. The function to convert back should take in the parameter names in any order and a numpy array and return a dictionary with them mapped back to right spot.