New SEIR process with FOI included

[ethho]

Addition of vaccination compartment depends on two separate FOIs with different beta values for vaccinated and unvaccinated individuals. The easiest way to implement this as of now is to call the Cython function _brute_force_FOI twice within a new Cython function similar to _brute_force_SEIR.

Work Plan

Work on this issue will be test driven.

• [x] New branch
• [x] New PR
• [x] Failing import pytest
• [x] New xs.process
• [x] Failing pytest for import of new models.basic model seir_with_foi
• [x] New model seir_with_foi
• [x] New pytest to include models.basic.seir_with_foi in TestToyModels.test_sanity
• [x] Failing pytest for models.basic.seir_with_foi in TestCompareBasicModels
• [x] New Cython function episimlab.seir.bf_cython_engine.seir_with_foi

[ethho]

Instead of including model as parameter in TestCompareBasicModels, I wrote a new pytest test_consistent_with_foi_separate.