Warning suppresssion

This makes the opm-parser module compile warning-clean with a very high warning level.

Relevant options used have been: -pedantic -Wall -Wextra -Wformat-nonliteral -Wcast-align -Wpointer-arith -Wmissing-declarations -Wcast-qual -Wshadow -Wwrite-strings -Wchar-subscripts -Wredundant-decl.

Notes:
 - The changes to Well.cpp only keep the existing behaviour, and do not consider the issues raised in #177, about treatment of `controlMode`.
 - The warning originally raised in EclipseGrid.cpp may indicate a real problem. I will submit a separate issue.
 - The function setupParserKeywordInt() in ParserTest.cpp seems to be unused. I will submit a separate issue.