Refs #10370 Added method to MDBox to reserve memory for events.

MergeMDFiles uses this method to reserve memory for events in an MDBox object. Removed reserve in loadAndAddFrom method on MDBox (could this have a detrimental effect on LoadMD?).
Refs #10370 Added unit test for ReserveMemoryForLoad.
Refs #10370 Reserve memory again in LoadMD.

Changes to loadAndAddFrom in MDBox meant memory is no longer reserved in LoadMD, this fixes the issue. Note the performance gain for this, if any, appears marginal.
Merge branch 'feature/10370_MergeMDFiles_memory_reserve' into develop