Changes from the previous release:
- No need of workaround for IBM XL 16.1 since CMake 3.11
- Add a reference implementation einspline_spo_ref next to einspline_spo
- Reduce the default number of MC steps from 100 to 5
- Format source with clang-format 7
- Enable hierarchy time
- Decouple the walker loop from the OpenMP parallel region
- Add a synchronous walker moving driver
- Fix several bugs