OutputWriters: remove unused classes and old parser in EclipseWriter

the APIs of "SimulatorOutput", "SimulatorOutputBase", "OutputWriter"
and "OutputWriterBase" are completely unused in opm-core, opm-autodiff
and opm-porsol. Since they also were in the way of the old parser
removal of the EclipseWriter, the eclipse writer class does not
inherit from them anymore. This means that these classes are now
dangling in the sense that they do not even have any child classes
anymore, but they so far, they are kept for legacy reasons.

this patch also gets rid of the old-parser-taking methods of
EclipseWriter. this avoids quite a few consistency issues and also
reduces the amount of duplicate work required. (if you want these two
items separated, ping me.)