allow `integrand_prototype` for oop integral functions

[lxvm]

Previously, IntegralFunction(f, integrand_prototype) would error for oop f and this pr makes it possible to pass an oop f because the integrand_prototype is still useful to infer the return type of f without evaluating it. Integral algorithms often need to have a prototype to have a correct wrapper so this will avoid additional integrand evaluations.

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 26.19%. Comparing base (d45b79a) to head (263f910). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #636 +/- ## ========================================== - Coverage 26.26% 26.19% -0.07% ========================================== Files 55 55 Lines 4283 4279 -4 ========================================== - Hits 1125 1121 -4 Misses 3158 3158 ```

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