tune_mtry: Tune mtry in final random forest.
In OHDSI/FuzzyForest: Fuzzy Forests

Calculates mse over different values of mtry, returns a plot displaying how the mse changes, and returns the value of mtry with minimum mse.

1	tune_mtry(object, X, y, step_size = 1, mtry_grid = NULL, main = NULL)

`object`	An object of type fuzzy_forest.
`X`	A data.frame containing the features selected by fuzzy forests. The names of selected features must be among the column names of X. If X is the training set, the resulting mse's may be biased.
`y`	The corresponding set of outcomes.
`step_size`	By default, mtry starts out high (equal to the number of features selected) and goes down by increments of size `step_size`. selected by fuzzy forests.
`mtry_grid`	A grid of values of `mtry` to test. If `mtry_grid` is non-null, the mse will be calculated for each specified value. In this case, the argument of `step_size` will be ignored.
`main`	Title of resulting plot.