refactor(dag): add NetworkX python package as DAG option

[jdhughes-usgs]

Modified makefile generation to remove date and time so that changes
to makefiles are limited to source file name and dependency changes.
Refactored setup.py to a more modern format. Bumped version number
to 1.2.1

[codecov]

Codecov Report

Merging #98 (f5de385) into master (2737095) will increase coverage by 0.020%.
The diff coverage is 91.176%.

@@              Coverage Diff              @@
##            master       #98       +/-   ##
=============================================
+ Coverage   65.899%   65.919%   +0.020%     
=============================================
  Files           16        16               
  Lines         4082      4105       +23     
=============================================
+ Hits          2690      2706       +16     
- Misses        1392      1399        +7     

Impacted Files	Coverage Δ
pymake/pymake_parser.py	31.578% <ø> (ø)
pymake/utils/_dag.py	81.967% <87.234%> (+1.103%)	⬆️
pymake/config.py	100.000% <100.000%> (ø)
pymake/plot/dependency_graphs.py	63.953% <100.000%> (ø)
pymake/pymake.py	75.077% <100.000%> (+0.237%)	⬆️
pymake/pymake_base.py	87.021% <100.000%> (-0.089%)	⬇️
pymake/utils/_compiler_language_files.py	95.833% <100.000%> (ø)
pymake/utils/download.py	68.299% <0.000%> (-1.153%)	⬇️
pymake/utils/usgsprograms.py	83.495% <0.000%> (ø)
pymake/utils/_compiler_switches.py	80.941% <0.000%> (ø)
... and 3 more