getpurevars: Identifies pure variables
In svkucheryavski/supure: Spectral Unmixing with Pure Variables

The method identifies indices of pure variables using the SIMPLISMA algorithm.

1	getpurevars(D, ncomp, offset = 5, exclude = NULL)

`D`	matrix with the spectra or their inverted second derivative
`ncomp`	number of pure components
`offset`	offset in percent for calculation of parameter alpha
`exclude`	which columns exclude when detecting the pure variables

The function returns a list with with following fields:

`ncomp`	number of pure components.
`purvars`	vector with indices for pure variables.
`purityspec`	matrix with purity values for each resolved components.
`stdspec`	matrix with weighted standard deviation values.
`purity`	vector with purity values for resolved components.