Always use the logical processor count for system max cores

[clintval]

I have an 8-core MacOS with hyperthreading (16 virtual cores total).

I'd like to tell my tasks to use them all!

Running ExamplePipeline before this PR:

[2021/01/23 22:40:21 | TaskManager | Info] Executing with 8.0 cores and 24g system memory.

And after:

[2021/01/23 22:45:31 | TaskManager | Info] Executing with 16.0 cores and 24g system memory.

Closes #370.

[nh13]

@clintval thanks for this PR, let us think about if we ant cores to be the # of physical cores, or the # of virtual cores/threads.

[tfenne]

I'm 👍 . I think I was the one who wanted physical cores originally, and back in the earlier days of hyper-threading it wasn't really clear to me how useful it was. However, nowadays my habit it to, e.g. run bwa on my mac with 12 threads, not 6. So I realized I'm doing this when running things manually.

If we wanted to we could always have a config setting for dagr.core_type=[logical|physical] but I don't think that's really necessary.