User defined pure variables index in mcrpure function should be used as is.

[svkucheryavski]

When user provides a vector with pure variables (parameter purevars), they should be used as is and computing of pure variable should be skipped (hint: remove line 382). Here is a code that reproduces this issue:

library(mdatools)
data(simdata)
spectra <- simdata$spectra.c
m <- mcrpure(spectra, ncomp = 3, purevars = c(10, 20, 30))
print(m$purevars)

The output should be 10, 20, 30.