Improve callout to reference match

[mash]

Improve callout to reference match by removing parentheses often seen in callouts.

As seen in the docs, callouts often include parentheses.
https://grobid.readthedocs.io/en/latest/training/fulltext/

I saw many occasions that these parentheses prevent matching the correct references.
See the red boxes when trying arxiv.org/pdf/1801.01290.pdf

[coveralls]

Coverage remained the same at 38.434% when pulling 11cbbcf on mash:callout-to-reference-author-match into ff10968 on kermitt2:master.

[kermitt2]

Many thanks @mash ! Very good finding, for some reasons I was thinking that the parenthesis were removed somewhere upstream in the process, but you're absolutely right.

In addition to your example PDF, here's the evaluation for callout resolution on the PMC set (1943 PDF - 139,835 reference callouts), branch update_header without your PR:

Precision citation contexts:     81.32
Recall citation contexts:        69.13
fscore citation contexts:        74.73

and with your PR:

Precision citation contexts:     81.41
Recall citation contexts:        69.86
fscore citation contexts:        75.2

Note that there are many references with numerical reference markers in PMC, so the positive impact should be much larger with arXiv for instance.