## Bring back FCLS

### Bring back FCLS

As pointed out in #1921 (closed) the NCLS algorithm we had (unmixing algorithm under Nonnegative Constrained Least Square) was buggy and was doing exactly the same as an UCLS (UnConstrained).

@terudel proposed to implement the FCLS algorithm (Fully Constrained) because:

[..] it is widely used in the hyperspectral community. It is a linear spectral unmixing algorithm that is based both on the non-negativity constraint of factors (such as NCLS) but also on the fact that the sum of the coefficients of the abundance map must be equal to 1 (which makes sense from a physical point of view).

Moreover, if we believed the code, we used to have the two algorithms FCLS and NCLS in the HyperspectralUnmixing application. So it would be nice to have at least one back.