Some work in proc_sphere_part

Summary

Various stuff in proc_sphere_part:

• improves documentation
• improves numerical accuracy by using EFT, which should reduce issues in case different components use YAC built's that were compiled by different compilers and/or different optimisation options

see also #271

Related Issues

• Closes #269 (closed)

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows Style Guide
• Documentation updated, if necessary
• WIP: Manual running of configure/build jobs was successful
• Update changelog, if changes are worth mentioning (Develop section in doc/src/changelog.dox, add section if not yet included)