'Returns basis vector for the provided index, if the index is a string.
If the index is a number, returns the coordinate variable correspon-
-ding to that index.'

def __getitem__(self, ind):

    if (not isinstance(ind, str)):
        if (ind < 3):
            return self.varlist[ind]
        else:
            raise ValueError('Invalid    index    provided')
    if (self.indices[0] == ind):
        return self.x
    if (self.indices[1] == ind):
        return self.y
    if (self.indices[2] == ind):
        return self.z
    else:
        raise ValueError('Not    a    defined    index')
