Check all variables in Patsy interaction term in estimate_coefficient method

[christopher-wild]

Describe the bug
If multiple variables are present in an interaction term (i.e. self.treatment = 'variable1:variable2'), each variable needs to be checked to be a type object. This is not currently implemented as it is not known how to return a list of variables from a Patsy formula neatly.

Additional context
First look into if Patsy can return a list of variables somehow, if not manual string splitting and iterating over them.

[jmafoster1]

I tried to run this with a more complex interaction term driving_score ~ I(completion_score * (1 - outside_route_lanes)) and it doesn't like it because the I basically just puts everything inside it into a block, so patsy doesn't recognise to pull out completion_score and outside_route_lanes as terms. It just has I(completion_score * (1 - outside_route_lanes)).

[christopher-wild]

Not closed with #264, PR changed scope