Corrected point-element distance estimate in
PotentialOperatorAcaAssemblyHelper.
Reverted to the old (faster, but less accurate) method of estimating
interelement distance in WeakFormAcaAssemblyHelper.

So we use again the distance between cluster bounding boxes rather than
minimum distance between the bounding boxes of elements belonging to the
active clusters.
Big refactoring of the code responsible for selction of quadrature rules.

This functionality has been extracted from local assemblers to separate
classes. It is now easier to customize quadrature rules at different levels
of complexity. See, especially, the documentation of
NumericalQuadratureStrategyBase.
Added a code (for debugging) showing how many integrals an integrator calculated.
Changed on of the constructors of NumericalQuadratureStrategy
(now accepts a const quadrature descriptor selector factory).
Added new constructors to Bempp::NumericalQuadratureStrategy and documented
it better.
Merge branch 'customized_integration' of https://github.com/bempp/bempp into customized_integration
Prevented potential evaluators from allocating too large arrays.
Made the constructor of AccuracyOptionsEx taking vectors work in Python.
Modified the assembler so that the distance between elements -- rather than
clusters -- is used to select the quadrature order.
Merge branch 'customized_integration'
Revert "Added a code (for debugging) showing how many integrals an integrator calculated."

This reverts commit 2d6bf7643fac38b9ee65a37c7809c45facd8eacd.