S3fit

1. Update emission line model frames to support electron density and temperature as input parameters.
2. Easier method to add/delete emission lines from the list, and to add/delete line tying relations.
3. Easier method to use a custom star formation history function.

**Full Changelog**: https://github.com/xychcz/S3Fit/compare/v1.5.1...v1.6

Minor updating
**Full Changelog**: https://github.com/xychcz/S3Fit/compare/v1.5...v1.5.1

First release of S³Fit.

**Full Changelog**: https://github.com/xychcz/S3Fit/commits/v1.5