Implementation of purity based method for spectral unmixing

[svkucheryavski]

Implementation of purity based methods (including SIMPLISMA), described in this book chapter: https://www.sciencedirect.com/science/article/pii/B9780444636386000036. The implementation should be similar to PCA and makes possible to project new spectra to the resolved model in order to obtain concentration values.

Following plotting methods should be implemented:

• plotSpectra() — show resolved spectra
• plotContributions() — show resolved contributions (concentrations)
• plotPurity() — shows purity plot
• plotVariance() — shows explained variance
• plotCumVariance() — shows cumulative explained variance

[svkucheryavski]

finished implementation and tests in current 0.11.0 branch (7b46544).