materialize() method for forcing draws and diagnostics (and inits and profiles) into memory

[MansMeg]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Separate materialize() from save_object() according to the discussion in #1179.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Måns Magnusson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.10%. Comparing base (cb6d218) to head (1fac734).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1181      +/-   ##
==========================================
- Coverage   91.23%   91.10%   -0.14%     
==========================================
  Files          15       15              
  Lines        6070     6091      +21     
==========================================
+ Hits         5538     5549      +11     
- Misses        532      542      +10     

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[MansMeg]

The failing test cannot be due to this PR. It seems to be a Windows issue in general.

[jgabry]

Thanks! Yeah we're seeing those failing windows tests in other places too, I'm not sure why yet. But definitely unrelated to this PR.