Function FiberIds.xyForCobras() assumes 0-indexed fiberIds

From the docString, the pfs_utils function FiberIds.xyForCobras assumes some 0-indexed cobra identifier, but the code actually assumes fiberId (but 0-indexed, not 1-indexed as the actual fiberId identifier is):

https://github.com/Subaru-PFS/pfs_utils/blob/1edf4cb1ac8d5552daed299acc12200c75add4e8/python/pfs/utils/fiberids.py#L229-L242

Accessing the x, y coordinates for a given cobra by fiberId is IMO useful. So I propose to keep the function, but change the docString to assume a 1-indexed fiberId, and correct the implementation such that it works with a 1-indexed fiberId.