Make BratReader more forgiving

[alaindesilets]

BratReader is currently very unforgiving. I propose to make the following improvements to it.

1) Create a BratRead without having to provide mappings.

• BratReader would already know mappings for the Annotations defined in the standard dkpro-core type system (ex: Person)
• If you provide a PARAM_MAPPING, those mappings would be ADDED to the default ones
• If the .ann file contains a label that is defined in neither of the default or PARAM_MAPPING mappings, it will use a "catch-all" Annotation type (NamedEntity for the moment, but could be something else)

2) Pass a directory or file to PARAM_SOURCE_LOCATION without having to worry about things like:

• Adding *.ann at the end of the directory path (the Reader would add it automaticaly)
• Making sure to pass the .ann file as opposed to the .txt file (the Reader would automatically converts it to .ann path)
• Making sure that the single file, or all the the .txt files in the directory have a corresponding .ann file (the Reader would automatically creates empty .ann files for orphan .txt files)

If this seems appropriate, I will create a feature request followed by a Pull REquest.

[reckart]

Since this is an enhancement and not a bug-fix, I'll target it to the 1.13.0 feature milestone and not to the 1.12.1 bug-fix milestone.

[alaindesilets]

I just realized that the latest changes I pushed break some tests in io-conll. I'll fix those and let you know.