There were functions in products.py that will be useful in other modules
but circular imports pop up if these functions don't have their own module.

_ACS_MISSING only gets used once (in a function that's now in utilities.py
so I moved this variable into that function. I don't think that these
missing values ever change

If one of the columns can' be converted, none of the columns get converted.

utilities._coerce used to take in an object and try to change the
entire object's dtype. Now, it changes only what columns can be
changeable.

For example, if there's a column of words and column of integers,
coerce cam now cast only the column of integers to type integer
and leave the column of words as pandas objects.

Add unit tests for coerce.

Add tests

replace_missing (the argument) had the same name as replace_missing
(the function)

This fix likely causes a lot of other issues with functions in this module.
Tests will probably be a good idea at some point here.

…nto rep_col_cases_114