implement c++ data archive

[pfrommerd]

should include a "record' task type that is associated with a context and you can control what variables are being recorded at what rates

this should be done similarly to show the devices are listed, with a "directory_scanner" task that monitors a directory for listed files and "open_archive" tasks that create associated "archive" containers when start() is called and destroy them when stop() is called.

archive creation should be supported through the create_context callbacks