New C++ API: Add templated getIndices method for OpTerm (#3073)

New C++ API: Add templated getIndices method for OpTerm (#3073)

* Implement templated getIndices method for OpTerm

* Add getIndices unit tests

* Update src/api/cvc4cpp.cpp

Co-Authored-By: Aina Niemetz <aina.niemetz@gmail.com>
* Update src/api/cvc4cpp.cpp

Co-Authored-By: Aina Niemetz <aina.niemetz@gmail.com>
* Add comment about DIVISIBLE_OP

* Update test/unit/api/opterm_black.h

Co-Authored-By: Aina Niemetz <aina.niemetz@gmail.com>
* Update test/unit/api/opterm_black.h

Co-Authored-By: Aina Niemetz <aina.niemetz@gmail.com>
* Update test/unit/api/opterm_black.h

Co-Authored-By: Aina Niemetz <aina.niemetz@gmail.com>
* Update test/unit/api/opterm_black.h

Co-Authored-By: Aina Niemetz <aina.niemetz@gmail.com>
* Add exception checks to other unit tests (instead of having its own function)

* Fix unit test names in opterm_black.h

* Add description to docstring for getIndices

* Formatting

* Clang format older commits

* Use '-' in docstring list to match other docstrings

* Support creating DIVISIBLE_OP with a string (for arbitrary precision integers)

* Move mkOpTerm(DIVISIBLE_OP, <str>) test to solver_black.h

* Fix pointer access

* Replace switch statement with if statement

* Guard string input for CVC4::Integer in mkOpTerm for consistency on GMP/CLN back-end