Allows multiple variables and dates in Output

Previously each output file is forced to contain one date and variable.
The framework is changed such that multiples of either are allowed. This
is an intermediate commit, the code is not tested.
Merge branch 'master' into flexOutput
Allow multiple configurations in one output

Output interface is made general enough such that any number of
configurations can be put in one file. The Netcdf, Verif, and Flat
formats don't take advantage of this.