Added EBL model, PEP8, prep Travis, rem. py2.7

[me-manu]

Several changes towards version 0.4:

• added a new EBL model (for EBL and optical depth) from Saldana-Lopez et al. (2021)
• Removed support for Inoue et al. 2013 EBL models (only included z=0, optical depth is still there)
• Removed python 2.7 support
• Changed default spline interpolation from quadratic to linear
• Added travis yaml and requirements files
• Added tests

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@287aebe). Click here to learn what that means. The diff coverage is n/a.

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