Add force_functor_cache option for Sprucing rate test
In the log files of Moore_spruce_rate_and_size
an error is produced:
ERROR: ld.so: object 'libtcmalloc.so' from LD_PRELOAD cannot be preloaded: ignored.
Adding force_funtor_cache
fix this, not sure if it solves lhcb/Moore#438 (closed).