Starttime in FilterCheckpoint

Issue/feature addressed

With this MR we introduce a "StartTime" parameter to the Checkpoint filter. This would allow the user to control the time step when to start writing the chk files.

Implementation

Define and make use of an additional variable, e.g. m_startTimeChk, to check whether the simulation time instant is >= m_startTimeChk: if (m_index++ % m_outputFrequency > 0 || time < m_startTimeChk).

Unit test

I will define a unit test for the incompressible Navier Stokes solver.

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).

This MR solves #304 (closed)

Notes

The unit test 234 fails, but it also fails for the master branch.