Enable cost model

[djelovina]

Initial version of the cost model.

[djelovina]

to test is

mkdir build && cd build
cmake -DGRB_ENABLE_TRACING=ON ../
make test_conjugate_gradient_reference_omp VERBOSE=3 #you should see -D_GRB_ENABLE_TRACING=1 definition
tests/smoke/conjugate_gradient_reference_omp /path/to/matrix_exmaple.mtx direct 1 1

[anyzelman]

Slotted for milestone 0.9