OMEROService: rename convert methods, again

I like renaming things!

Seriously, the overloaded method name would have once again caused
problems with the addition of an "omero.client client" argument to the
toOMERO conversion method. I also like the new names because they are
unambiguous about which direction things are being converted.
DefaultOMEROService: promote TODO to FIXME

We want to get image parameter types working ASAP.
OMEROService: pass OMERO client when converting

For 'toImageJ' we need it to extract pixels from the OMERO server, and
for 'toOMERO' it is necessary to upload new pixels to the server.
DefaultOMEROService: improve javadoc
OMEROService: reorder API methods

I prefer the non-client-specific methods first, followed by the
conversion methods requiring an omero.client object after that.
OMEROService: add pixels upload + download methods

These methods "convert" between ImageJ Datasets and OMERO pixel IDs.
DefaultOMEROService: pass client when converting

We will need the OMERO client to download pixels.
DefaultOMEROService: lean on downloadPixels method

When converting a POJO to the needed ImageJ type, we can leverage the
OMEROService's downloadPixels method if the POJO is a number and the
ultimately needed type is an ImageJ Dataset.
DefaultOMEROService: lean on uploadPixels method

When converting an ImageJ Dataset to an OMERO-compatible format, we can
leverage the OMEROService's uploadPixels method to upload the pixels,
then pass the resultant pixels ID to OMERO.
DefaultOMEROService: simplify private method name

I think I just won't be happy unless there is a method
*somewhere* in this project called 'convert'.

This method is really just a helper of the 'toImageJ' method; it could
all be inlined, but it would make things longer and probably harder to
understand, so I'm leaving it separated for the time being, since as we
all know: "A method should only do one thing."
DefaultOMEROService: tweak identity logic

When the convert method is given an object whose type is actually
compatible with the requested type, we were short-circuiting, doing an
immediate cast. But in the interest of future expansion, we may not
always want to do that. The code later which invokes ClassUtils#convert
will also handle this case just fine, so we can rely on that for now.
DefaultOMEROService: add extremely helpful comment