1. Hide SPECULAR_DATA_TYPE inside ISpecularScan
2. Add some validity checks
3. Change limits for qz vector

For future use in python

ISpecularDataHandler --> ISpecularScan
SpecularDataHandlerAng --> AngularSpecScan
SpecularDataHandlerQ --> QSpecScan

Correct Doxygen info on interference functions

Switch to absolute value of difference in GISAS plot observer for bet…

Switch to absolute value of difference in GISAS plot observer for better comparison with original images

CMake: Uniform lowercase in CMake commands

Add helper class for debugging slicing of MultiLayer (can be used later for unit tests)

Change version number to development number (1.15.99)

Merge release branch

Add fitting test for reflectometry with q-defined instrument

Update version number, changelog and author list

Co-Authored-By: waltervh <walter.vanherck@gmail.com>

Co-Authored-By: waltervh <walter.vanherck@gmail.com>

Fix parameters in fitting example

Regenerate doxygen and Python API

Implement position variance in 3d view (#2282)