Avoid use of reserved names.

The page at https://github.com/LLNL/sundials/blob/develop/doc/shared/sundials/SUNContext.rst describes the Context structure as declared as follows:

struct _SUNContext *SUNContext

This is unfortunate: Both C and C++ reserve all names that (i) start with an underscore, followed by an uppercase letter, (ii) start with two underscores. See https://en.cppreference.com/w/cpp/language/identifiers .

I haven't checked whether that's the only name in SUNDIALS that has this problem, or whether that's a pattern. Either way, it's probably worth avoiding the issue, and because these names aren't part of the public interface, it shouldn't even be that difficult to rename things if you so chose.

The reported location is doc/shared/sundials/SUNContext.rst; start there, then search the repository for similarly reserved internal names in C and C++ declarations. Done means the affected internal names no longer use the reserved patterns and the documentation reflects the resulting names.