fixes bug in routine yac_interp_weights_add_wsum

Summary

Routine yac_interp_weights_add_wsum used an absolute value to remove irrelevant weights. However, in case of interpolation method SPMAP with scaling: invtgt, it could happen that all weights for a target fell under this limit. Therefore, it is now replaces with a relative tolerance value.

Related Issues

• Closes #246 (closed)

Documentation

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 successfull
• Update version info, if changes are worth mentioning (Develop section in doc/src/version_info.dox, add section if not yet included)