Add custom source loading

[shyamd]

This PR enables adding a custom source file such as .py or .ipynb file. The only requirements are as follows:

1. The source file has to be in a subfolder from where mrun is called. This is a limitation of the pathfinding that has to convert a module import command into a path
2. The custom python file has to include either a __builder__ or __builders__ defined somewhere in the file. This is what gets loaded by mrun to run.

Addresses #277

Examples:
mrun -n 2 -v builder_test.py
mrun -n 2 -v builder_test.ipynb
mrun -n 2 -v main_builders.json builder_test.py

[lgtm-com]

This pull request introduces 2 alerts when merging 828727b into b45d769 - view on LGTM.com

new alerts:

• 2 for Unused import

[mkhorton]

This is going to be so useful :)

[lgtm-com]

This pull request introduces 1 alert when merging 5e74854 into b45d769 - view on LGTM.com

new alerts:

• 1 for Unused import

[codecov]

Codecov Report

Merging #278 into master will increase coverage by 0.19%.
The diff coverage is 92.38%.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   88.30%   88.49%   +0.19%     
==========================================
  Files          27       28       +1     
  Lines        2026     2121      +95     
==========================================
+ Hits         1789     1877      +88     
- Misses        237      244       +7     

Impacted Files	Coverage Δ
src/maggma/cli/source_loader.py	91.95% <91.95%> (ø)
src/maggma/cli/__init__.py	93.47% <92.30%> (+1.17%)	⬆️
src/maggma/cli/multiprocessing.py	94.59% <100.00%> (+0.04%)	⬆️
src/maggma/stores/aws.py	82.50% <100.00%> (ø)
src/maggma/stores/gridfs.py	89.20% <100.00%> (ø)
src/maggma/stores/mongolike.py	93.62% <100.00%> (ø)

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[lgtm-com]

This pull request introduces 3 alerts when merging 62ee2c9 into 6ba742b - view on LGTM.com

new alerts:

• 2 for Unused import
• 1 for Module is imported with 'import' and 'import from'

[lgtm-com]

This pull request fixes 1 alert when merging f10552d into 6ba742b - view on LGTM.com

fixed alerts:

• 1 for Unused import