Make sure cell params and initial conditions are parsed consistently

[finsberg]

There are currently some inconsistency in the way parameters and initial conditions are parsed as noted in #25

There are currently 3 ways so set the cell parameters; either using a dictionary with values, using a file called drug_factors_file or using a file called popu_factors_file. If you pass in a parameter using the dictionary and then pass the same parameter in one of the files, then the parameter in the file will overwrite the one in the dictionary.

For the initial conditions it is the other way around. In that case, the parameter in the dictionary will overwrite the parameter in the file.

In my opinion this can potential lead to unexpected behaviour, and I think the best would be to only make it possible pass a dictionary. I think the parsing of the files should be done at a higher level.