Memoise get functions for caching within session

[ateucher]

[ateucher]

This is difficult to implement when parsing filter expressions in bcdcget_geo_data(x, ...)

I had to take it out for now because memoization forces evaluation of its arguments, so it fails before it can parse the filter expressions into CQL. Eg.,:

bcdc_get_geodata("bc-major-cities-points-1-2-000-000-digital-baseline-mapping", 
                               POP_2000 < 1000)

because there is no object called POP_2000 so when that argument is evaluated it throws an error.