Calculate mesocrystal FF exactly (#510)

The MR implements "exact" computation of mesocrystal formfactor, so that the result is equal to composition of the same contents.

Both approaches, old "approximatate" (ReMesocrystal::reciprocalSpaceSum) and new "exact" (ReMesocrystal::realSpaceSum) do coexist. One of them can be chosen by setting the option SimulationOptions::setUseMesoReciprocalSum (GUI doesn't have this option yet). Now the old way is set by default for the compatibility with current tests, but later this will be changed.

The same algorithm (Mesocrystal::calcBasisIndexes) is used to count particles for both "exact" computation and 3D visualisation.

In principle, this solves #510 (closed), but the result should be proven and anchored by adding corresponding tests comparing mesocrystals with compositions and single particles.