Replace instances of @submodel with to_submodel()

[penelopeysm]

This PR replaces remaining instances of @submodel with to_submodel in DynamicPPL, including in the demo models.

To get tests to pass, it also implements the functionality needed for the pointwise_logp family of functions to work on submodels.

To prevent @submodel from being completely untested, It also adds some very basic tests for @submodel inside test/deprecated.jl, just to make sure that it's still behaving correctly.

Closes #750

Closes #752

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.93%. Comparing base (62a0f19) to head (5fa5410).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
- Coverage   85.94%   85.93%   -0.02%     
==========================================
  Files          36       36              
  Lines        4276     4280       +4     
==========================================
+ Hits         3675     3678       +3     
- Misses        601      602       +1     

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[coveralls]

Pull Request Test Coverage Report for Build 12323480116

Details

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• 31 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.03%) to 86.015%

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Files with Coverage Reduction	New Missed Lines	%
src/submodel_macro.jl	2	81.08%
src/varinfo.jl	3	84.92%
src/model.jl	5	75.93%
src/threadsafe.jl	21	46.61%

Totals
Change from base Build 12323466233:	-0.03%
Covered Lines:	3678
Relevant Lines:	4276

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💛 - Coveralls