Refactor model: DataItem and SpecularDataItem

See merge request !64

fix values of FTDecayFunctionVoigt 1D & 2D

See merge request !62

1D: without the fix m_eta refers to the same as m_decay_length => calculations will be wrong
2D: without the fix m_eta refers to the same as m_decay_length_x => calculations will be wrong
2D: the ctor values have been in wrong order => clone will be wrongly initilaized

Refactor model

See merge request !61

dependency inversion to move CoordSystem1D down in include hierarchy

See merge request !60

add a bayesian tutorial

See merge request !57

not supported under Windows