fix: pptx rendering

[erdos]

Investigating https://github.com/erdos/stencil/issues/160

Issue seems to be that data model in model.clj is too rigid, cannot properly represent pptx files. The internal references are missing from the generated files.

TODO:

• [x] Make sure both slide, slideMaster, slideLayout files are present in the generated pptx file.
• [x] Ensure that there are no dangling references that are included in the template.
• [x] Guarantee that the XML namespaces are consistent in the internal xml files by fixing the alias names.
• [ ] Unit test coverage
• [ ] Visual integration test to prevent regression
• [x] Test manually with LibreOffice
• [ ] Test manually with MS Office (365?)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.39%. Comparing base (65d7d1b) to head (92f781f).

Files	Patch %	Lines
src/stencil/model.clj	94.73%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
+ Coverage   93.24%   93.39%   +0.14%     
==========================================
  Files          31       31              
  Lines        2072     2088      +16     
  Branches      102      100       -2     
==========================================
+ Hits         1932     1950      +18     
  Misses         38       38              
+ Partials      102      100       -2     

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