Allow for tagging an initialization problem to ODEFunction/DAEFunction

[ChrisRackauckas]

This will need downstream support in the solvers to override the use of an initializer.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 26.27%. Comparing base (1774538) to head (5b80eb8).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/scimlfunctions.jl	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
+ Coverage   26.25%   26.27%   +0.01%     
==========================================
  Files          55       55              
  Lines        4269     4281      +12     
==========================================
+ Hits         1121     1125       +4     
- Misses       3148     3156       +8     

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