[si] remove parts of the code depending on fgsl, we no longer use it
[si] drop custom boundary conditions, they were never used
[si] remove ugly code responsible for introducing inner cutoff in density distribution. It has not been used in ages
[si] remove unused parameters
[si] calculate grav acceleration in a local procedure. This allows to remove ugly bits calling gravity functions and messing with multigrid potential
[si] remove yet another useless runtime var
[si] reorder vars in read_problem_par
[si] summing potential in custom grav_pot is no longer required, since we are not using it directly during initialization
[si] fix rbuff size for gauss
Merge pull request #27 from Xarthisius/si_clean

Cleanup in streaming_global