implement to_writer method and clean up RustCycle csv serde

[kylecarow]

• adds a to_writer method
  • RustCycle just needs to override to_writer instead of to_file
  • Cleanup/reorg of SerdeAPI-alike functions in RustCycle (write_csv etc)
  • New RustCycle::to_csv function to be consistent with to_yaml etc, same functionality as RustCycle::to_str("csv") (which now calls RustCycle::to_csv)
• All SerdeAPI errors are now exposed as IOError/OSError in Python rather than RuntimeError

I noticed the Python-exposed RustCycle::from_csv loads a cycle from a given filepath, rather than a string like from_yaml, from_json, etc. I think this is something that I would definitely want to change in FASTSim 3. @calbaker what are your thoughts about making all these string deserialization methods have _str at the end (starting in FASTSim 3, to avoid API changes)?

i.e. from_yaml would become from_yaml_str to make it clear that it takes a YAML-formatted string, rather than a filepath

[calbaker]

@kylecarow , is this ready to merge?

[kylecarow]

@calbaker Left a comment for you to ponder, but this PR is ready to merge, yea