[Amibe] Add an abstract distance metric class

This new class is created in order to extend the definition of a
distance metric. Since it is hard to extend directly the DistanceMetric
class, this new abstract class is created in order to limit the
duplication of code. DistanceMetric is now a subclass of
AbstractDistanceMetric.
[Amibe] Numeric metric: geometric interpolation on first interval
[Amibe] Add new metric for a priori mesh refinment around singularities

DistanceMetricInterface requires an extra parameter alpha, hence unused
by DistanceMetric.
[Amibe] use SingluarMetric in remeshSingularity.py