Speed up SimPathsModel.getProcessed

[andrewbaxter439]

Some things to try out

• Lazy lookups when multiple joins are needed - joining household->benefitUnit->members seems to be very slow for example. Perhaps need to adjust this
• Indexes may help lookup in some cases of SQL? Particularly in populateDonorTaxUnitTables where lookup by systemYear=" + systemYear; is very slow
• Parse column types as decimals on reading in, rather than altering afterwards?

[andrewbaxter439]

Closed as fixed as detailed in #261 and #267