Proper Deprecation of Eliminate Methods

[varunagrawal]

Found this while looking at elimination in GaussianFactorGraphs for MHS with @ProfFan. Let's get this out of the way so we don't have accidentally cause a dependency on this.

Fixes #889

[ProfFan]

@varunagrawal Could you also add the deprecation pragmas?

#if defined(__GNUC__) || defined(__clang__)
#define GTSAM_DEPRECATED __attribute__((deprecated))
#elif defined(_MSC_VER)
#define GTSAM_DEPRECATED __declspec(deprecated)
#else
#define GTSAM_DEPRECATED
#endif

[dellaert]

@varunagrawal Could you also add the deprecation pragmas?

#if defined(__GNUC__) || defined(__clang__)
#define GTSAM_DEPRECATED __attribute__((deprecated))
#elif defined(_MSC_VER)
#define GTSAM_DEPRECATED __declspec(deprecated)
#else
#define GTSAM_DEPRECATED
#endif

I would think that is a different PR. That would be cool though

[varunagrawal]

I had to look up what that was but yeah it would be awesome to have that and mark the corresponding methods with it. Next PR 🙂