Added parameter to not use the Julia worker pool

[gbruer15]

By default, JUDI uses all workers in the current pool. This MR allows the user to specify to run serially even when there is a worker pool.

[mloubout]

Can you add a simple test in that checks the setter and the default are correct.

https://github.com/slimgroup/JUDI.jl/blob/master/test/test_basics.jl

Looks goood

[codecov[bot]]

Codecov Report

Patch coverage: 83.33% and project coverage change: -0.01% :warning:

Comparison is base (77ca35e) 83.78% compared to head (17aff3d) 83.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #200 +/- ## ========================================== - Coverage 83.78% 83.78% -0.01% ========================================== Files 28 28 Lines 2436 2442 +6 ========================================== + Hits 2041 2046 +5 - Misses 395 396 +1 ``` | [Files Changed](https://app.codecov.io/gh/slimgroup/JUDI.jl/pull/200?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slimgroup) | Coverage Δ | | |---|---|---| | [src/JUDI.jl](https://app.codecov.io/gh/slimgroup/JUDI.jl/pull/200?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slimgroup#diff-c3JjL0pVREkuamw=) | `81.35% <83.33%> (+0.22%)` | :arrow_up: |

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