[Amibe] String is needed by remesh.py --record flag
[Amibe] Fix description of the point metric file format
[Amibe] Add a MetricSupport.AnalyticMetric abstract class which defines getTargetSizeTopo

Copy the definition of getTargetSizeTopo from DistanceMetric, and let DistanceMetric
extend AnalyticMetric instead of AnalyticMetricInterface.
[Amibe] Use the new AnalyticMetric abstract class

AnalyticMetricInterface had been extended, it now contains an extra
getTargetSizeTopo method.  One must either define both getTargetSize
and getTargetSizeTopo, or use AnalyticMetric.

Note also that an extra argument had been added to getTargetSize.
[Amibe] Minor changes in remesh.py

Use the debug_write_counter variable to enable/disable debug mode,
so that one does not have to edit the writeVTK method.