Warning seriously (using Logging.jl and with user settings)

[Kolaru]

With the emergence of the NG flag and similar considerations, it seems like implementing a proper logging system is becoming increasingly pressing.

Amongst the thing that we would want to optionally warn the user are:

• Creating an interval with NG
• Creating an interval with decoration that is not :com
• Creating an interval from floating point numbers with @interval (related to New macro for guaranteed operations #627)
• Using a function that is not tight (like power which has a much slower tight version)
• Creation an invalid interval
• Creating an empty interval

Depending on their needs, users will probably be interested in enabling or disabling different warnings.

The logging will need to include the full stracktrace in most case, for example to quickly pinpoint where a computation lose its guarantee.

Logging.jl probably allow to do all that.

Looking at the list (please comment if you see more case that could do with a warning) it seems like most of the warnings would fall into the interval constructors, so it may not be that hard to do.