POM: bump to pom-imagej 3.1

Among other things, this bumps the version of scijava-common to 2.35.0,
giving us access to the org.scijava.util.ReflectedUniverse utility
class, which we will make use of shortly.
Remove compile-time usages of SCIFIO

Some I/O-related DatasetService methods exposed SCIFIO types
(specifically: io.scif.Metadata and io.scif.config.SCIFIOConfig) in
their API. Those references have been generalized to Object. This will
break backwards compatibility mostly for those calling "save" and
expecting the return value to be typed as Metadata rather than Object.

The DefaultDatasetService employed SCIFIO in its canOpen, canSave, open
and save methods. That usage has been wrapped via reflection, so that if
SCIFIO is present on the classpath, the methods will continue to work as
before, while avoiding a compile-time dependency on SCIFIO. This
approach is temporary, until those deprecated methods are removed in
favor of a better solution.

See also:
https://github.com/scijava/scijava-common/compare/robust-io
POM: remove no-longer-needed SCIFIO dependency

The io.scif:scifio project will now depend on imagej-common, instead
of the other way around. The rationale is that the ImageJ library
(net.imagej:imagej-common) is the place the adds metadata richness on
top of the ImgLib2 library (net.imglib2:imglib2). And the SCIFIO
library (io.scif:scifio) is built on top of that metadata richness.

For details, see:
http://imagej.net/Architecture
Remove the DatasetIOPlugin

It is migrating to SCIFIO:
https://github.com/scifio/scifio