Potential fix to modelReport failure with Random Forest and Boosted Regression Trees models

[lidefi87]

Response plots (univariate and marginal) are now produced with Random Forest and Boosted Regression Trees models. This addresses issue #36.

I performed checks using devtools and this was the result:

── R CMD check results ────────────────────────────────────────────────────────────────────────────────────────────── SDMtune 1.3.1 ────
Duration: 10m 29.6s

❯ checking package dependencies ... NOTE
  Package suggested but not available for checking: 'zeallot'

0 errors ✔ | 0 warnings ✔ | 1 note ✖