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

add bug for scalfmm.png location

This fixes the following error:

error: constexpr variable 'r_max' must be initialized by a constant
expression