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Cleanup and fix minor issues in tests and classes related to interpolation based FMM + Update gaussian correlation kernel to include derivatives.

Add
  - CutOffKernel which uses the matrixkernel class in the P2P Operator
  - CutOffAlgorithm to explain how we can use scalFMM with only cutOff method. The Octree dos not contain any local or multipole values.

The `particle` pointer was allocated in the forEachLeaf callback but
freed outside. If there was more than one leaf, the pointer would not be
freed before the new allocation.

	- The FAQ is available from dixygen web pages
       - add comments for epsilon parameter in Chebyschev kernels
      ...
To continue.

Non C++11 conforming compilers have problems when a static const class
variable does not have its out of class definition.

Add the SCALFMM_INVERSE_PRIO env variable to reverse the priorities (for schedulers different from  heteroprio)

Adding a scalfmm_execute_only_far_field method to the API, in order for a user to compute a FMM without P2P

Fix typos