levelset: rework overall design

The main purpose of the rework is to reduce the memory footprint of the
levelset. To achieve this goal, the values computed by the levelset are
stored in separate caches. Each cache can be enabled separately form the
other and can operate in one of the following modes:

    values can be cached as their are evaluated;
    values can be pre-computed inside the narrow band;
    values can be pre-computed on the whole domain.

When no caches are enabled, the only memory used by the levelset is the
memory that each object uses to store its own information (for example
the object that evaluates the levelset of a segmentation stored
information about the normals of the segmentation).

When no cached are enabled, the evaluation of the levelset is thread
safe and this allows to use the levelset object inside parallel OpenMP
regions.

This new levelset aims to be 100% compatible with the existing levelset.
Some functions are now deprecated, but thay should still work as before.

examples/RBF_example_00001.cpp

@@ -244,7 +244,7 @@ void run(std::string filename,
     levelset.setPropagateSign(true);
     levelset.setSizeNarrowBand(sqrt(3.0) * h);
     // Compute the levelset
-    levelset.compute(id0);
+    levelset.fillCache(id0);
     // Write levelset information
     mesh.write();
     bitpit::log::cout() << "Computed levelset within the narrow band... " << std::endl;

src/levelset/bitpit_levelset.hpp

@@ -42,9 +42,8 @@
 
 #include "levelSetObject.hpp"
 #include "levelSetProxyObject.hpp"
-#include "levelSetCachedObject.hpp"
+#include "levelSetCache.hpp"
 #include "levelSetSegmentationObject.hpp"
-#include "levelSetSignedObject.hpp"
 #include "levelSetBooleanObject.hpp"
 #include "levelSetMaskObject.hpp"