Sambuca-core

------------------
* forward model changes:

* renamed slope_cdom to a_cdom_slope to indicate that it is an absorption
value, and for consistency with the main naming convention.
* renamed slope_nap to a_nap_slope for the same reasons.
* split backscatter slope into two values, one for phytoplankton, and one for
NAP. Both are optional, and if the NAP backscatter slope value is not
supplied, the phytoplankton backscatter slope value will be reused. This
preserves the behaviour of the IDL model which only has a single backscatter
slope value for both phytoplankton and NAP. The new slope parameter names
conform to the primary naming convention.

------------------
* Additional SIOP and IOP outputs added to forward model. These values were
being calculated anyway, and they were required by Bioopti.
* Docstrings are now almost complete.
* Some naming inconsistencies have been addressed.

------------------
* Data loading functions for sensor filters, and general spectra implemented.
Supported formats are ENVI spectral libraries, and tabular data in Excel and
CSV formats.
* Full physical model added to the forward model.

------------------
* Initial commit of core Sambuca code after split from single package.