GUI: crash on assigning specular instrument with existing non-uniform axis to a dataset with another number of points

1. Add specular instrument
2. Switch its axis type to uniform OR assign it to some dataset.
3. Assign specular instrument to anothe dataset (of different size) ==> CRASH:

Assertion m_values.size() == m_frame->size() failed in /home/svechnikov/Projects/bornagain/Device/Data/Datafield.cpp, line 42