Add a method for reporting warnings

[tomjnixon]

Ideally it should be possible to report a problem without throwing an exception, and without just printing to stderr.

Some alternatives:

1. use some 3rd party logging library which has a system for controlling where the output goes

• advantage: easy
• disadvantages: not very flexible, probably not re-entrant

2. pass around an error handling callback

• advantage: re-entrant, flexible
• disadvantage: potentially a lot of work

3. store a global error-handling callback

• advantage: simple and flexible
• disadvantage: not re-entrant

IMO it would be really good to have a "warning hierarchy" as in libear, to allow users to decide what to ignore, and what to turn into errors. This rules out option 1.

If 2 was implemented nicely, it might be possible to make messages really nice by adding context, like: "while parsing ABF_XXX: invalid time format 'yyy'".