Add tree diameter data, stepwise term removal, and more

[bpbond]

@hanioid10414 This PR does several things:

• Adds the tree diameter data we talked about
• Adds "basal area" (the conducting area of the tree stem, a function of diameter squared) to the linear models)
• Users stats::step() to automatically remove non-significant terms from the models
• Collects results from the species-specific models into a single data frame:

# A tibble: 3 × 5
  Species Model_R2 Pred_mae  Pred_R2 Model_terms                                                   
  <chr>      <dbl>    <dbl>    <dbl> <chr>                                                         
1 LITU      0.0540   0.116  -0.00338 (Intercept), wx_par_den15, BA                                 
2 ACRU      0.0744   0.127   0.0174  (Intercept), soil_vwc_15cm, wx_par_den15, BA, soil_vwc_15cm:BA
3 FAGR      0.169    0.0830  0.0866  (Intercept), wx_par_den15, BA