add a class for log messages of the parser

unsurprisingly, it's called ParserLog. For now, it is not used very
extensively, but it allows to demingle the log messages from the deck
objects. (i.e., it's possible to pass a const pointer to the deck
object to e.g. EclipseState and one will still get additional log
messages.)
replace the deck-internal logging system by ParserLog
add separate unit test for the Opm::checkDeck() function
EclipseState: significantly extend usage of the new logging facility

in the process quite a few exceptions have been converted into log
messages.