[reference model] Add precise mode

This adds --precise_mode=1 option to tosa_referece_model,
which will cause reference model to convert all floating point tensors
to FP64 tensors and compute all operators accordingly.

Also adds optional -p arguments to test runners tosa_verif_run_tests.py
and tosa_verif_framework_compiler_runner.py to run tests in precise mode

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: I156055216ad61710096497a8fa1a653be2a602a3