Faster cavity reshape

Hello Christophe and other Gmsh devs,

This little update of Hxt include an optimized reshapeCavityIfNeeded function (reshapes a cavity into a star-shaped cavity suitable for inserting a point). That function had a O(n^2) computational complexity, and it should now be O(n) 🎇. In most cases, the cavities are small and the number of tetrahedra that this function remove is even smaller. However, in some cases like in the first tests of issue #1123 (closed), big cavities were make Hxt hang seemingly forever.

This update also brings a fix to "walking2cavity": in some extremely rare case, the walk could last forever.