ENH: Add support for multiple metrics and regularization

[Leirbag-gabrieL]

Hi, apparently it was not possible to do a multi-image registration with a transform metric like this:

(Metric "AdvancedMattesMutualInformation" "AdvancedMattesMutualInformation" "AdvancedMattesMutualInformation" "AdvancedMattesMutualInformation" "TransformBendingEnergyPenalty")
(Metric0Weight 1.0)
(Metric1Weight 1.0)
(Metric2Weight 1.0)
(Metric3Weight 1.0)
(Metric4Weight 10.0)

(NumberOfHistogramBins0 32)
(NumberOfHistogramBins1 32)
(NumberOfHistogramBins2 32)
(NumberOfHistogramBins3 32)

Now it is possible :)