Change name 'chunk' by '.chunk' in inst/chunking folder

Hi @aho!

I have look in all functions (R folder) if chunk() was appearing instead .chunk() but I think all functions are fine.

I have also look in all file under inst/chunking. Do you mind to take a general review to confirm we have finished with this change?

Thanks in advance,

Núria

PS this merge request should solve #69 (closed)