Disable unquoted string literals

With the next major release we should disable unquoted string literals. Currently we cannot distinguish between a variable name and a string literal. Something like

CONST STRING fmap = "fieldmpas/someFM.T7";
cav1: RFCavity, L=0.2927, VOLT=60, FMAPFN=fmap, ...;

currently doesn't work because fmap in the second line is considered to be an uppercase string instead of the name of a variable.

In order to (try to) not break any input files we should add string constants for all those string attributes that only accept a set of predefined strings. Examples for such attributes are the attribute FSTYPE of the FIELDSOLVER command which accepts FFT, FFTPERIODIC, SAAMG and NONE or the attribute EMISSIONMODEL of the DISTRIBUTION command which accepts NONE, ASTRA and NONEQUIL.