Support undefined head in CoNLL-U

[martinpopel]

According to the CoNLL-U specification, Underscore () is used to denote unspecified values in all fields except ID._
However, Read::CoNLLU emits warnings "Argument "_" isn't numeric in array element at .../Udapi/Core/Document.pm line 129" when reading files with underscore in the HEAD column.

My suggestion is

• attach nodes with _ as HEAD to the technical root (as if they had 0 as HEAD)
• mark somewhere (misc) that the dependency is actually undefined, so when writing the file, we can reconstruct the _ (and not substitute it with 0)

I am not sure if the second point is worth the work. We should benchmark the speed of CoNLLU reading, so it is not slower for regular files.