GTSAM_EXPORT consistency implementation

We are trying to bring this repository in-line with GTSAM's export requirements for Windows DLLs. We need a PR to more uniformly implement the `GTSAM_EXPORT` keyword across the repository.

See: (from Prof. Frank Dellaert)
The new public classes and free functions should be annotated with `GTSAM_EXPORT` so they are exported correctly from shared-library builds on Windows. The GTSAM contributing guide documents the required pattern here: https://gtsam.org/notes/contributing/#using-gtsam-export

More detail here: https://github.com/borglab/gtsam/blob/6c59503fea13bb9025b4f098bb3412e57b3531c2/Using-GTSAM-EXPORT.md

Start by reading the GTSAM contributing guide section on using GTSAM_EXPORT and Using-GTSAM-EXPORT.md, then inspect the repository for public classes and free functions. Done means a pull request that applies the documented GTSAM_EXPORT pattern consistently for Windows shared-library builds across the repository.

Written by the indexing model from the issue text.