Added class NeutronBk2BkExpConvPVoigt.  Refs #7653.
Refactor codes.  Refs #7653.

Moved some methods belonged to ThermalNeutronBk2BkExpConvPVoigt to
IPowderDiffPeakFunction because they are used by
NeutronBk2BkExpConvPVoigt, too.
Checkpointing work.  Refs #7653.
Checkpointing work.  Refs #7653.
Moved some common function to interface.  Refs #7653.
Complete coding (almost).  Refs #7653.
Refactor E1() to interface.  Refs #7653.
Checkpointing work. Refs #7653.
Fixed a bug during testing this ticket.  Refs #7653.
Added some unit test (incomplete).  Refs #7653.
Checkpointing work.  Refs #7653.
Merge branch 'master' into feature/7653_add_fp_peak9
Added some unit tests.  Refs #7653.
Merge branch 'feature/7653_add_fp_peak9' into develop