Added sub_type to Pe_tw

Can be used to distinguish between sample and buffer measurements
Removed debug from DataClass

It doesn't belong there
Test suite for Pe. Added checking of variables in super_absorptive
Test suite for pe_merge
init undersampling to 0

for fast scanning, undersampling is zero anyway
testing of merge...
tests for dataclass, improved setter for zeropad_to
killed variable _zeropad_by. It is now only a setter/getter

Less to keep track of!
Some more killing of _zeropad