Framework introduction

[TonioF]

In this branch, I have renamed the class Prior to PriorCreator, and all sub-classes accordingly. A PriorCreator has to implement two methods:

@abstractmethod
def compute_prior_file(self) -> str:
    """
    Might perform some computation, then retrieves the path to a file containing the prior info
    :return:
    """

@classmethod
def get_variable_names(cls) -> List[str]:
    """
    :return: A list of the variables that this prior creator is able to create priors for
    """

compute_prior_file replaces the method formerly known as RetrievePrior, the latter simply returns a list of variable names.
This approach eliminates all explicit mentions of soilmoisture prior or vegetation prior in the prior engine. It has got quite a few renamings, so please check whether you agree with them. Also, it will probably clash with the user_prior branch, so we might want to merge that one first.

[tramsauer]

triggering a new build via re-opening.
build error seem to be related to: ContinuumIO/anaconda-issues#9449

[tramsauer]

I made the get_variable_names method also an @abstractmethod as this should also be mandatory for each new prior_creator. see #31