Handle default units in concrete IUnitConverter (do not rely on it being the same as the underlying units of the original detector)

Use converter's default units instead of DEFAULT + revert Python example because of failing PyPersistenceTest

Refactoring in Fourier transformation machinery

# Conflicts:
#	auto/Wrap/libBornAgainCore.py
#	auto/Wrap/libBornAgainCore_wrap.cpp

Thanks for the changes!

Redmine: #1860

This reverts commit 5018d27b.

Redmine: #1860

Redmine: #1860