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.
Revert "Refs #10370 Reserve memory again in LoadMD."

This reverts commit c63a2a2144232cf8e1a9354fc2e8e00e3d9abcd3. Instead of this we can still allow reserve to be called in loadAndAddFrom method in MDBox.
Refs #10370 Added reserve back to loadAndAddfrom in MDBox.

This means the reserve will still be performed if required, but not if memory was already reserved.
Revert "Refs #10370 Added reserve back to loadAndAddfrom in MDBox."

This reverts commit 54ad8c6ae6f51e6e2710a400c2856ff6a5fcba36. Decided to leave the reserve as previously, as push back should allow for adequate performance in most cases even if another method relying on loadAndAddFrom in MDBox does not call reserveMemoryForLoad first.
Revert "Revert "Refs #10370 Reserve memory again in LoadMD.""

This reverts commit ec902c6e993b6c61a8c550ce22eafb5962ea8da5.
Merge branch 'feature/10370_MergeMDFiles_memory_reserve' into develop