Make use of default variable store in multiply_core

[tramsauer]

get default variables from multiply_core/variables/variables.py.
e.g. in soilmoistureprior creator:

try:
    defaultvariables = multiply_core.variables.get_defaul_variables()
except:
    defaultvariables = [sm, lai, cab, ...]

or so... just to avoid duplicity in the multiply code

[tramsauer]

possibly

default_variables = [d[short_name].lower() for d in multiply_core.variables.get_default_variables()]

.lower() should not be necessary..

[TonioF]

Very nice, I approve! Just in case, you can also define your own variables if that will be necessary. I guess that it can come in handy when users submit their own priors.