Spirit

A number of bugfixes... And:

**Core**
- Refactored into self-contained library "Spirit"
- The python and julia bindings are now contained in core
- **Working CUDA backend** with a speedup of ~20 (note that the visualisation currently causes IPS drops after some iterations)
- Changed Configurations setters API (now any configuration can be inserted at any position and with various position filters, e.g. spherical and cylindrical regions)
- Added Unit Tests (including Python API!)
- Added mutex locks to prevent crashes when setting configurations etc. while a simulation is running

**QT User Interface**
- Added boundary condition indicators
- Added optional shadows for isosurfaces
- Added light direction sliders
- Added colormap rotation and inversion settings
- Added "bounded" camera for more intuitive camera movement
- Added visualisation of effective field
- Improved StatusBar infos
- Added and improved key controls (e.g. camera movement)
- Added slab visualisation modes
- Added screenshot functionality
- Smooth FOV slider

**Core**
- Switched from own math functions to Eigen3.3 - this improved the performance of the code by a factor of around 20-40 (depending on host, configuration etc.)
- Improved core structure
- Initial CUDA backend
- Initial unit tests
- Improved Logging and Output
- Added input log
- New Quadruplet interaction in Hamiltonian_Anisotropic

**UI-QT**
- Persistent settings
- Pretty infos in status bar
- More user-friendly UI, less non-working buttons
- Initial try for OSX App Bundle

**UI-Python**
- many bugfixes in core module
- added several methods in core module

- Replaced the gl library with the improved VFRendering library: https://github.com/FlorianRhiem/VFRendering
- Added a few UI capabilites, such as minimizing the control bar, controlling spin size, ...
- Added surface visualisation (still unfinished)
- Added IPS information to status bar, improved output
- Added choice of scalar type and other CMake improvements

- Added Plotting of Energy data (including interpolated values) using QTCharts, requiring QT5.7
- Added Isosurface visulisation and other improvements
- The MMF method now works for the single-spin case
- More API functions, better separation of core and UIs
- Cleaner thirdparty usage, including qhull as external project

This release should generally work, but graphs and the MMF method are missing and there might be a few bugs.