Work out different queries for 'getting one value' and 'getting all values'

[albertmeronyo]

Now grlc supports dropdown lists for possible parameter values.
Howerever, sometimes users don't want to specify one single value for these parameters, but all of them.
This would imply mad-overkill query rewriting if done via parameters, so best solution is to write two types of queries:

• One using parameters to fix one value
• One using VALUES(?x IN (?v1, ... , ?vn)) to specify several (possibly all) values

[albertmeronyo]

@AshkanA This is related to CLARIAH/grlc#13