Add --workspace feature

[ErikBjare]

We have a need for a "workspace" in tests/evals, but I find that it could be a generally useful feature to let each log potentially have a workspace subfolder where project files can reside.

This subfolder would be created by providing --workspace to gptme, or detected on log load.

[ErikBjare]

I implemented not quite this in #100, perhaps I should reconsider the implementation.

[ErikBjare]

Redid the implementation so that one can now pass a magic @log workspace and it will create a workspace in ./<logdir>/workspace, as well as default to using any such workspace folder if it exists in the log folder upon load.