Issue a warning when numeric variables are passed in formulas (ind. vars, Error terms and grouping terms)

[mjskay]

Seems a possibly common mistake to pass numerics to Error() when factors are intended. Might be worth a warning, with an arg to disable.

[mjskay]

Or more generally when passed as independent variables

[mjskay]

Errors on fixed effects were already addressed previously, and grouping terms for lmer don't need this error (as it addresses this problem anyway), so just need to address the problem for Error() terms.