fix (and simplify) centroiding logic
fix (and simplify) centroiding logic

(without sacrificing performance)

the current algorithm can be summarized as follows:
1) go across the mass range using a fixed step
2) maintain a flag showing whether there's any peak around
   - 'around' means +/-12 * sigma (12 is in fact an overkill)
3) if the flag is set, skip envelope calculation
4) otherwise, calc. envelope & check if the m/z is a local maximum
   - here distant peaks (in the same sense) are also ignored

SpatialMetabolomics/SM_standalone,SpatialMetabolomics/SM_standalone,SpatialMetabolomics/SM_standalone