Spectral

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
---------

* [Issue 113] `orthogonalize` was broken in release 0.21.

Changes
-------

* As of version 0.21, Spectral Python (SPy) is released uder the MIT License.

* Switched most console `print` statements to logging statements for finer
control of module output.

* Cleaned up a lot of code to put imports at module (file) level.

Bug Fixes
---------

* [Issue 99] matplotlib's "cmap" keyword now works again with `spy.imshow`.

* [Issue 102] Creating a `SpectralLibrary` no longer fails when some
optional band parameters are not provided.

New Features
------------

* Support for ECOSTRESS spectral library (supercedes ASTER library).

* Accept "mask" and "bg" keywords in `imshow` and `get_rgb` (affects color
scaling).

* Denoising methods added to `PrincipalComponents` (issue 91) added by
gemmaellen.

* Added "raw" to list of checked ENVI data file extensions.

Changes
-------

* Increase speed of `spectral_angles` computation by ~30x using `einsum`.

* Eliminate `tostring` deprecation warnings by migrating to `tobytes`.

Bug Fixes
---------

* Support parsing ENV bbl values as floats (issue 67).

* Removed "scale" keyword to handle TransformedImage objects in
`envi.save_image` (issue 83).

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