parameters: make it possible to group them

as a test, the parameters for the newton controller are now specified
within the [Newton] group. e.g. instead of

NewtonWriteConvergence = true

you have to now specify

[Newton]
WriteConvergence = true

in your INI file. the GET_PARAM macro then reads

GET_PARAM(TypeTag, bool, WriteConvergence, Newton)

instead of

GET_PARAM(TypeTag, bool, NewtonWriteConvergence)

In both cases the compile time default value is retrieved from the
NewtonWriteConvergence property.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6477 2fb0f335-1f38-0410-981e-8018bf24f1b0