Optimize exponents summation in "exact" mesocrystal FF algorithm (#892)

Now the summation of exponents uses the analytical expression for the geometric sum along 3rd axis (z axis by default).

So the computational complexity now is ~O(Nx * Ny) instead of previous ~O(Nx * Ny * Nz), where N is the number of particles along the axis.

struct ShapeIndexes is introduced to diminish number of variables and to mask the unused ones in some contexts.

Resolves #892 (closed)