ServiceHelper: simplify getService method calls
ServiceHelper: use correct generic parameter

All we know about the Class in question, at this juncture, is that it
*extends* Service, not that it *is* Service. So we should use
"Class<? extends Service>" rather than just "Class<Service>".

Of course, due to runtime erasure of generics, it does not actually
matter that much, but it's nice to get things right...
WIP: ServiceHelper: change how service deps are filled

TODO: write description here
Add tests for Context creation with services

We check that contexts get created (or not) in various situations,
with service dependencies of various natures.
WIP: Inject context and service parameters

TODO: Write description.
Add tests for @Parameter injection mechanism

We want to make sure that *any* class which has @Parameters of type
org.scijava.service.Service or org.scijava.Context has those parameters
populated with the context's associated values, when
Context#inject(Object) is called on it. We also want to make sure that
*if* the object implements the Contextual interface, the result is
exactly the same when calling Contextual#setContext(Context).