Extend "proximity" command to be able to compute the proximity value;
Implement classes for calculate proximity value between two shapes;
Add possibility to set up the number of sample points for the input shapes;
Add tests lowalgos/proximity.
Several methods in Selection classes have been moved to header files for better inlining.
BVH_Constants - added new enumeration defining common constant values used with BVH trees.
BVH - replaced NCollection_Handle with Standard_Transient handle
in classes BVH_Properties, BVH_Builder, BVH_Tree, BVH_Object.
Defined global BVH-builders instead of allocating a new one for each object set.
SelectMgr_ViewerSelector - added new method ::SetEntitySetBuilder() defining
default BVH Tree builder for SelectMgr_SensitiveEntitySet.
Added new method SelectMgr_SensitiveEntitySet::SetBuilder()
for overriding default BVH tree builder.
