Renderer per clock + fix to decorator and context manager + refacto

[victorbenichoux]

This PR is relatively large, and achieves the following:

• Add the capability to have different formatting strings for different clocks, which adresses the requests in Suppress line numbers in tock() #12 as well as feature request: easier control on renderer #4
• Simplifies the format string, with a general name field that works in decorators/context managers as one would expect
• Refactor by splitting ticktock.timer into ticktock.clocks with clocks, ticktock.collection with collection, ticktock.timers for decorator/contextmanager interfaces
• Refactor by removing largely unused Clock and tick parameters (e.g. tick_time_ns)
• Make clock identity consistent by never disambiguating clocks on their names
• Better tests of context manager and function decorator functionality
• numerous improvements to the documentation