Add sparse matrix compression benchmarks

Here is what I get on my computer:

./bench_sparse
2019-07-17 17:12:07
Running ./bench_sparse
Run on (4 X 3408 MHz CPU s)
-------------------------------------------------------------------
Benchmark                            Time           CPU Iterations
-------------------------------------------------------------------
convert_dense_to_mapped      167378757 ns  167410714 ns          7
convert_dense_to_compressed   11542329 ns   11439732 ns         56
convert_dense_to_coordinate   12055664 ns   11562500 ns         50

that is about 100 FPS for one core.