Fix compiler warnings

[dweindl]

Intel compiler complains about:

• include/amici/vector.h(263): warning Sparsify dwdp to reduce computation time for adjoints #858: type qualifier on return type is meaningless
• include/amici/sundials_matrix_wrapper.h(115): warning Sparsify dwdp to reduce computation time for adjoints #858: type qualifier on return type is meaningless

[thomassligon]

Interesting, this really does refer to the Intel compiler, and not the Microsoft compiler. The description of the warning is here:
https://software.intel.com/content/www/us/en/develop/articles/cdiag858.html
which says:

Cause:
Compiler generates this warning when it finds a type qualifier applied to the return type of a function. C++ allows the type-qualifier to be const or volatile , but either qualifier applied to a function return type is meaningless, because functions can only return rvalues and the type qualifiers apply only to lvalues.

Resolution:

1. Remove the type qualifier from the return type of the function.
2. This diagnostic can be safely ignored.