Add validation checking on column and length arguments

[ha0ye]

The core C++ code will crash when trying to access memory that it doesn't have, such as columns beyond the limits of a matrix or values past the end of a vector.

We can add some simple checks to the R wrappers to check for these to produce an error that won't crash RStudio.

One MRE:

dat <- rnorm(83)
pred <- c(43, 83)
lib <- c(1, 42)
crash_out <- simplex(dat, lib, pred, tau = 1:10)

[ha0ye]

Some of this is now done in #29, I am unsure about the column checking.