Improve UX

Depends on https://gitlab.com/micro-ROS/ros_tracing/ros2_tracing/merge_requests/101

Closes #10 (closed)

General:

• Display progress percentage when processing. It works really well and provides some feedback, which is good to have when processing big traces.
• Remove references to "pickle file", and instead use the term "output file" or "converted file."
• Improve README by describing how to analyze a trace step by step and providing a quick Python example.
• Display time differences (total time when running commands) depending on the value (e.g. 400 ms vs. 6 s vs. 3 m 10 s).

CLI:

• Make convert optionally take an output file name (to be created under the given trace directory) instead of a full path (default output file name is "converted" but we can change that of course).
• Provide a ros2 trace-analysis command with convert and process verbs (as a more generic interface).

Using the commands:

• Make convert step optional by having the process command check if the given path is a file. If it is, it tries to import it just like before. If it is a directory, it checks if there is a "converted" file directly inside it, otherwise it tries to import the path as a trace directory. If that fails, it gives an error message. Thus you can provide process with a path to:
  • a trace directory, or
  • a converted file
• Provide a --force-conversion/-f option for process to re-convert a trace even if a converted file is found.