Set n_workers for dask LocalCluster

[talumbau]

• Explicitly set n_workers for LocalCluster creation (through Client). If this is not set, for machines with a high CPU count, the number of workers will be much larger than 7. In client.scatter with broadcast=True, this can cause hangs due to how dask distributed works. It's presumed that this bug is only apparent in more recent runs on machines with high CPU counts, although that is not confirmed. But, this fix does allow the example to run to completion on a machine with CPU_COUNT=128.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c0802c6) 73.76% compared to head (6843966) 73.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #85 +/- ## ======================================= Coverage 73.76% 73.76% ======================================= Files 24 24 Lines 1616 1616 ======================================= Hits 1192 1192 Misses 424 424 ``` | [Flag](https://app.codecov.io/gh/PSLmodels/OG-USA/pull/85/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PSLmodels) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/PSLmodels/OG-USA/pull/85/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PSLmodels) | `73.76% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PSLmodels#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jdebacker]

Looks great and ran well on my machine. Merging. Thanks @talumbau!