Move compilation code, deprecate constructing a model without ever compiling

[WardBrian]

The pre-2.0 part of #695

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

This moves the compilation code outside the model class and issues deprecation warnings for cases we'd like to disallow in 2.0, namely the use of a CmdStanModel without an executable at all.

I think if we get this in, I can update #691, and then we can release 1.2.0

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):

Simons Foundation.

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)

[codecov-commenter]

Codecov Report

Merging #700 (19ac74e) into develop (13fe59f) will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #700      +/-   ##
===========================================
+ Coverage    79.78%   79.86%   +0.08%     
===========================================
  Files           75       75              
  Lines        11316    11367      +51     
===========================================
+ Hits          9028     9078      +50     
- Misses        2288     2289       +1     

see 13 files with indirect coverage changes

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[mitzimorris]

LGTM! cleaner!