Elements of slopes are different

The data (a list of possibly) within the `slopes` element of the `sim_slopes()` function, are of different class, the first one is of class "data.frame" and the second one is of classes "matrix" and "array". This is probably to the two moderators or the modx.values.

This prevents the uses of apply() functions.

`out <- sim_slopes(model = reg.res,
pred = X,
modx = W,
mod2 = Z,
modx.values = "plus-minus",
mod2.values = "plus-minus",
jnplot = TRUE`

Start by reproducing the two-moderator `sim_slopes()` call from the issue and inspect how the `slopes` element is assembled. Compare the classes of its entries for the `modx.values` and `mod2.values` settings, then verify that the resulting list has a consistent structure usable with `apply()`.

Written by the indexing model from the issue text.