New Features (thanks to kormang)
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* Continuum computation and removal
* See functions `spectral_continuum`, `remove_continuum`, `continuum_points`
* Support for USGS spectral library (`spectral.USGSDatabase`)
* SMACC algorithm for endmember extraction (`spectral.algorithms.smacc`)
Bug Fixes
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* [Issue 113] `orthogonalize` was broken in release 0.21.