Don't pass virtual sources to Bio-Formats
Don't pass virtual sources to Bio-Formats

Bio-Formats doesn't handle non-existant sources gracefully. Currently it
throws a FileNotFound exception (especially given that we have removed
the .fake format from our ImageReader).

So we need to check if a source is virtual or not. If so, the
BioFormatsFormat checker should return false.

Closes #6

scifio/scifio-bf-compat