Fixes seqfish reader v2

[LucaMarconato]

Additional fixes to support the seqFISH v2 file format.

Changelog

• improved parsing of OME XML metadata
• automatically parsing images as multiscale when they are larger than a threshold
• allowing arbitrary strings as ROIs prefixes
• now representing different ROIs in different coordinate systems
• parsing z coordinate for points
• fixed bug with indices of shapes when multiple rois were considered

CC @LLehner @adamdingliang