[Resolves #27] Improve api in class init

[dsleiter]

• require keyword-only arguments in init functions to make class instantiation more explicit
• improve init docstrings
• move update_docval utility function to a utils module and improve the docstring to better explain usage

I also tried to remove the need for init params columns and colnames in containers that derive from DynamicTable since those are defined at definition time, however, I discovered those are used when building the container from disk, so they need to remain.

[dsleiter]

Awesome, thanks for the review @rob-luke!