Move "def from_X" module methods to classmethods of Demographics and use cls()

[ckirkman-IDM]

This is to allow proper class inheritance in emod_api/emodpy/emodpy-malaria class hierarchy. Right now, emod_api from_XYZ() functions only return emod-api Demographics objects even when called from emodpy-malaria layer (should return an emodpy-malaria Demographics object).

emod_api.demographics module contains functions e.g.:

def from_template_node(lat=0, lon=0, pop=1000000, name="Erewhon", forced_id=1):

They all build Demographics objects and should be located inside the Demographics class as @classmethod .

All related tests will need to be updated.

emodpy-hiv will be affected as well as emodpy-malaria