Updated wrappers on new Fixed Relaxation API and get L2 norm functions - Towards MUI v2.1

[Wendi-L]

What has been done

• Updated C, Fortran and Python wrappers on the new Fixed Relaxation API (residual calculation functions).
• Implemented Fixed Relaxation get under relaxation factor and Fixed Relaxation get residual L2 norm function for C, Fortran and Python wrappers.
• Updated Changelog with newly implemented features.

  Demos

• Updated demos can be found in v2.1 branch of Wendi Liu's personal MUI-demo fork. It will be pushed to the MUI-demo official repository with the upcoming MUI v2.1 release.