Ebltable

This release fixes issue 6 .

This release fixes a minor bug when reading in the optical depth values from the Finke et al. (2022) models. It also features a new notebook that demonstrates how the `ebltable.tau_from_model.OptDepth` class can be interfaced with a spectral model from `gammapy`.

This new release contains several new features and improvements:

- The EBL model from Saldana Lopez et al. (2021)
- The EBL model from Finke et al. (2022)
- The EBL model from Franceschini et al. (2017)
- improved testing
- A new base interpolator class (PR 5 ) from which the optical depth and EBL classes inherit their base functionality. This drastically reduces code duplication
- Issue 3 addressed the extension of the tau tables to zero for z = 0 which improves the interpolation behaviour

Added Python 3 compatibility and the Franceschini et al. (2017) gamma-ray opacities.