Add warnings if reader encounters unknown categories or items

[benmwebb]

python-ihm was originally designed to read and write only the subset of PDBx/IHM mmCIF that viewers and modelers are interested in, and so the reader silently ignores categories or items it doesn't know about. However, if we want to use the library for any purpose where we need to extract all the data from the file (e.g. for remediation) it would be helpful for the reader to warn about missed data. Add an (optional) flag to ihm.reader.read to turn on these warnings.