otbcli_HyperspectralUnmixing give negative results with NCLS

Description

The abundance maps obtained using the option NCLS (Non Negative Constrained Least Square) can be negative. There is no difference with UCLS (Unconstrained Least Square) whatever the chosen band.

Steps to reproduce

Use both Cuprite and Pavia datasets. Endmembers are extracted using otbcli_VertexComponentAnalysis. Unmixed images are obtained using otbcli_HyperspectralUnmixing.

Configuration information

Ubuntu 18.04, OTB 6.6.1