Add missing imports for gcc 13

Missing #include <cstdint> in Stochastics.h and StochasticsInterface.h when compiling with gcc 13

https://gcc.gnu.org/gcc-13/porting_to.html

The following headers are used less widely in libstdc++ and may need to be included explicitly when compiling with GCC 13:
[...]
<cstdint> (for std::int8_t, std::int32_t etc.)