[boost] [MultiIndex] proposal - delayed index initialisation and maintenance

I am just sort of throwing this out there as a wild idea. Having a trait on any given index such that it is only initialised upon first access. The motivation being - When displaying a table of rows of fields, the user may not necessarily click on any given column header for sorting, and would only rarely click on the majority of column headers. Therefore it is not required to maintain any given column index unless the user has requested sorting by that index. Neil