Unify PDMat and PDSparseMat + move SparseArrays support to an extension
#262
| Job | Run time |
|---|---|
| 2m 22s | |
| 3m 0s | |
| 2m 6s | |
| 2m 31s | |
| 2m 54s | |
| 3m 14s | |
| 2m 28s | |
| 2m 56s | |
| 3m 15s | |
| 3m 29s | |
| 4m 25s | |
| 32m 40s |