Fix #165 Use Corrected CFL Criteria

A. Taflove & M. E. Brodwin
IEEE Trans. on Microwave Theory and Techniques, August 1975
Formula (7)
Show Optimal CFL in InitialiserController

- shows selected and minimum possible ratio for time step
- refactor to use log<picLog::PHYSICS> for standard sim
  output during startup
Add support for math::tan
Fix checkpointing in HDF5Writer

 - rename default data file
 - new default name for checkpoint files
 - create both data and checkpoint for same timestep
Merge pull request #374 from ax3l/topic-mathTan

Add support for `math::tan`
Merge pull request #371 from ax3l/fix-cfl

Fix #165 Use Corrected CFL Criteria
Merge pull request #375 from f-schmitt-zih/fix-checkpoints

Fix checkpointing in HDF5Writer