This is a test case for Vlasov translation operator.

It initializes a spherical distribution in spatial space near the
origin in a periodic box. The distribution is translated with constant 
speed to +x,+y,+z direction for roughly one period (dynamic
timestepping).

This test case tests for errors
- Vlasov translation
- dynamic timestepping
- sparse velocity machinery

This test case does not test errors in
- vlasov acceleration
- boundary conditions
- field propagators

You can compare the density 'rho' or the velocity block data 'proton'
against the reference values.

