Initial Splitting Follow on Fixes

[Owen Arnold]

1) ConvertToMD::performInitialSplitting should not have a hard-coded number of boxes. Although 50 is likely to remain as the default, It worries me that we cannot change this without recompiling and distributing mantid. Can you change this to be a user property which defaults to 50?

2) The initial splitting is Off by default, Nick's opinion is now that it should be ON by default. In the code use true/false rather than 0/1 to set default values. It's just a coincidence that 1 == true and 0 == false in c++, makes for more readable code if you don't mix boolean and integer types.

3) Unit tests should be made to run fast. If you cannot get the time down < 1 second, for this test method, then write it as a system test.

[Nick Draper]

Moved to R3.5 at the R3.4 code freeze

[Stuart Campbell]

This ticket has been transferred to github issue ​12159