v0.3.1 JOSS feedback

This release addresses some feedback and bugs found during the course of the JOSS review.

What's Changed

• Create CODE_OF_CONDUCT.md by @gully in #91
• Create CONTRIBUTING.md by @gully in #92
• Make sure that rv shift is not double counted. by @gully in #95
• Astropy 5 dropped support for python 3.7, exclude it in build matrix by @gully in #96
• Updates to normalize() by @kfkaplan in #81
• Adding method to sort stitched spectra by wavelength by @ericasaw in #85
• Methods for EchelleSpectrumList now act on and return a copy of the s… by @kfkaplan in #87
• Possibly a fix for #94-- unit issue stemed from ancillary data miscue by @gully in #97
• Standardize paths to example data in the tutorials, and (for now) always re-render notebooks with nbsphinx by @gully in #98
• Overhaul tutorials to use HTML paths, also use nbstripout by @gully in #99

New Contributors

• @ericasaw made their first contribution in #85

Full Changelog: v0.3.0...v0.3.1