NRRD: defer helper reader initialization

Instead of initializing the helper reader in setId (where we don't need
it), wait until openBytes is called for the first time.
Andor SIF: reduce the number of readLine() calls

This brings the setId time down to ~0.5s.
Varian: reduce the number of calls to readLine()

This brings the setId time under 0.5s.
Molecular Imaging: reduce the number of readLine() calls

setId time is now under 1s.
Zeiss CZI: reduce setId time for files with uncompressed images
InCell: setId performance improvements

In particular, reduce the number of loops over core.size(), and the
number of objects instantiated in said loops.