Evaluate allodb equations at dbh 1, 50, and 100

[maurolepore]

Relates to ropensci/allodb#16

source

Right now/ to start (and maybe this is all we'll ever want), it may be worth writing a very simple script where each equation will be evaluated at ~3 dbh values (e.g., 1 cm, 50 cm, 100 cm; counting only those within range of the equation's DBH limits) and equations flagged if they don't fall within a range that we consider reasonable for that size (@gonzalezeb could provide guidance on those threshholds).