Simplify serialization to use a simple zip file

[meissnereric]

Description of changes:
Serialization was messy before, and required users to keep track of some 6 files on their system. This change makes it so they only have to carry around a single zip archive, and all of the other structure is contained within that zip archive. It's never unzipped except in memory during the loading process and no files are written to memory except the entire zip archive.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #153 into develop will decrease coverage by 0.12%.
The diff coverage is 88.97%.

@@             Coverage Diff             @@
##           develop     #153      +/-   ##
===========================================
- Coverage    85.19%   85.06%   -0.13%     
===========================================
  Files           78       78              
  Lines         3850     3898      +48     
  Branches       654      660       +6     
===========================================
+ Hits          3280     3316      +36     
- Misses         376      383       +7     
- Partials       194      199       +5

Impacted Files	Coverage Δ
mxfusion/util/__init__.py	100% <ø> (ø)	⬆️
mxfusion/models/factor_graph.py	84.72% <100%> (-0.17%)	⬇️
mxfusion/util/serialization.py	85.71% <85.71%> (ø)
mxfusion/inference/inference_parameters.py	84.4% <91.66%> (-4.49%)	⬇️
mxfusion/inference/inference.py	83.33% <93.02%> (+1.51%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 96ccde7...ad9e6dd. Read the comment docs.