Use a single TableProxy class

The distinction of Table/Column proxies was based on backward
compatibility with the (columns, summary) tuple return type. It is now
much more convenient to return a proxy for the columns directly and have
a property .summary to allow accessing the summary data if needed.
Append newline to each log record (~ MAD-X command)
Restrict Model API to model functionality

Also, expose madx instance as public variable from Model to make up for
the fact that the model itself now only exposes functions on the model.

Note the distinction between Model.has_sequence()/get_sequence_names()
and Madx.has_sequence()/get_sequence_names(). The former check what is
defined in the model, while the latter check what is actually known to
the MAD-X instance.