Merged requested to merge develop-performance-r into develop
This PR is a first step in increasing the performance of pure reflectometry calculations. It addresses one issue related to the numerical computation and adds the computation of pure reflection coefficients that avoid unnecessary computation and copy of many coefficients. There is a substantial performance gain for small layer stacks: This PR does not address the following issues:
- For large stacks, there is still the quadratic scaling due to the parameter tree.
- The multi-threading engine seems not to perform very well for small numbers of Q-points under certain operating systems.
- Computing intensities of multiple channels for polarized systems requires the repeated and identical computation of R and T amplitudes.