tables: make them instantiable exclusively by EclipseState

since tables can be tricky, we now enforce that the compiler bails out
if the user tries to instantiate a table class manually.

Note that a bit of trickery is needed to keep the low-level unit
tests working...
tables: remove trailing underscores from the protected and private methods

opm-parser does not follow this convention...
move the table classes from .../Utility to .../EclipseState/Tables