Make LoadEventPreNeXus faster

[peterfpeterson]

Original Owner: @jzikovsky

This is a continuation of http://trac.mantidproject.org/mantid/ticket/3224.

Profiling still says that the majority of the time spent is in processing the list rather than reading from disk by at least a factor of 100. The next big target is finding a better way of allocating memory in the individual event lists (via reserve) rather than just using the defaults and letting vector re-allocate memory as necessary.

[peterfpeterson]

This issue was originally trac ticket 3244