family and order argument optional in variables.py

[amna-ukaea]

before:
class Variable took in order and family:
def init(self, name = "", order = 1, family = 1, block = "")

now:
class Variable doesn't:
def init(self, name, order=None, family=None):
self.order = Order(order) if order is not None else None
self.family = Family(family) if family is not None else None

"family" isn't always needed in an input file and can be auto assigned in MOOSE.

benefit:
it makes it easier to run an input file sometimes by letting MOOSE auto assign the order and family based on the variables' dependencies.