Factorization of the searchForInitiations functions

Most of the lines in the implementation of the searchForInitiations functions of DamageGrowthMesh and DamageGrowthOctree are similar. They have been put in a findInitiationPoints() function, which returns a vector of mPoint, corresponding to the points where a new damaged zone has to be inserted. This vector is empty if no initiation is necessary. Note that this works for both multiple and single search for initiations (in the last case, the function returns a vector with 0 or 1 mPoint). After getting the points where new initiations are necessary, the searchForInitiations functions of DamageGrowthMesh and DamageGrowthOctree perform operations which are specific to these formulations.

Commit done from an idea of A. Salzman.

This commit also implements multiple search for initiations to Octree computation (see commit 9650851e for more details about multiple search for initiations.). Fixes #7 (closed).