Converting third-party modules to submodules

Redocumented all of the functions. However, I still recommend that we move the modules in `thirdparty/` to [submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules), as they provide an upstream and will likely lead to less licencing issues.

_Originally posted by @travisdriver in https://github.com/borglab/gtsfm/pull/258#discussion_r705631936_

Start by inspecting the modules under `thirdparty/` and the linked Git submodules documentation. Read the discussion in pull request #258 to recover the context and identify the upstream repositories involved. Done means the third-party modules are represented as submodules and the licensing concerns raised in the issue are addressed.

Written by the indexing model from the issue text.