Scalability break-down in `NNLSSolver` for more than 2 processors

Scalability test using the unit test `tests/test_NNLS` in `par-nnls` branch shows a significant slow-down when more than 2 processors are used:

| Number of processors  | Time     |
| :---        |          ---: |
| 1      | 382ms  |
| 2     | 249ms   |
| 3     | 10902ms   |
| 4     | 15296ms   |

This test result is from a m2-chip macbook. 

This issue might be related to #269 .