`Microsimulation.df` throws `TypeError` with some sequences of variables

[MaxGhenis]

This works:

from openfisca_us import Microsimulation
sim = Microsimulation()
sim.df(["state_code", "snap_gross_income_fpg_ratio"])

but this doesn't:

sim.df(["snap_gross_income_fpg_ratio", "state_code"])

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
[<ipython-input-11-856cb2e0dc06>](https://localhost:8080/#) in <module>()
----> 1 df = sim.df(["snap_gross_income_fpg_ratio", "state_code"])
      2 ca_below_fpl = df[(df.snap_gross_income_fpg_ratio < 1) & (df.state_code == "CA")]
      3 ca_below_fpl

3 frames
[/usr/local/lib/python3.7/dist-packages/openfisca_tools/microsimulation.py](https://localhost:8080/#) in map_to(self, arr, entity, target_entity, how)
    216                 return entity_pop.project(arr)
    217             if how == "mean":
--> 218                 return entity_pop.project(arr / entity_pop.nb_persons())
    219         elif entity == target_entity:
    220             return arr

TypeError: ufunc 'true_divide' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

state_code is a household-level string, snap_gross_income_fpg_ratio is a spmu-level float.

[nikhilwoodruff]

OK, so this is because the spm_unit -> household mapping works for the string type, but not the other way around. Will need some logic added to the map_to function.