Handle deprecation and removal of arithmetic conversion for enums

[ManifoldFR]

Issue description

In C++26, the implicit conversion to underlying types for anonymous enums in operations (such as bitwise ops) will be removed. Right now there is a compiler warning about the deprecation, since at least Clang++14 according to what I've seen online.

Relevant issue on Eigen's GitLab: https://gitlab.com/libeigen/eigen/-/issues/1826

• This was fixed with this MR on Eigen last year, by explicitly converting anonymous enum values to int

One of the warnings I'm getting is this:

/path/to/pin/repo/include/pinocchio/context/generic.hpp:54:83: warning: bitwise operation between different enumeration types ('Eigen::StorageOptions' and 'pinocchio::context::(unnamed enum at /path/to/pin/repo/include/pinocchio/context/generic.hpp:47:5)') is deprecated [-Wdeprecated-anon-enum-enum-conversion]
   54 |     typedef Eigen::Matrix<Scalar, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor | Options>
      |                                                                   ~~~~~~~~~~~~~~~ ^ ~~~~~~~

Pinocchio contains the following offending code in the context headers:

pinocchio/include/pinocchio/context/generic.hpp

Lines 77 to 83 in d1e1f68

	namespace context
	{
	typedef PINOCCHIO_SCALAR_TYPE Scalar;
	enum
	{
	Options = PINOCCHIO_OPTIONS_DEFAULT
	};

And in some other classes I think.

Suggested change

Since these are compile-time constants, we can switch them to something like

namespace context
{
  // ...
  static constexpr int Options = PINOCCHIO_OPTIONS_DEFAULT;
}

in generic.hpp, and perhaps some using statements in other namespaces and other static constexpr int [...] declarations in classes.

We could also adopt the solution from the MR on Eigen but I think these anon enum values to int everywhere would be tedious.