PLOT: added default plot modification/replacement

If a custom plot is defined, which uses only one value, it will replace
the default plot for the respective value.

For example:
Default metric plot: 'DegreeDistributionR.DegreeDistribution'
Custom plot MD2 will replace it:
MD_D2_FILENAME = distributionPlot1
MD_D2_VALUES = DegreeDistributionR~DegreeDistribution
GEN: added BatchGeneral-interface
PLOT: removed redundant plotting code!