component filename, default fmt ascii, revert sim path -> workspace

[wpbonelli]

• move filename field to Component. this required supporting kw_only on xattree classes (passed thru to attrs.define) which lets us put required/optional fields in any order. otherwise attrs dictates required fields must come first therefore an optional filename on Component would mean no required fields on concrete component types.
• set the default load/write format to ascii on Simulation
• rename Simulation.path back to workspace as it was originally.. introduce an intermediate component subclass Context which is a component with a workspace
• make path a derived property on Component and Context