Small improvements in Calocalibclustermomentsmaker2

Small improvements in Calocalibclustermomentsmaker2:

• make MyCellInfo inherit directly from std::vector<std::pair<int, double> > instead of having one member
• use std::vector::insert instead of for loop in MyCellInfo
• use std::array for the vectors when the size is known in compile time.
• use inline initializers
• Move the code of get_calib_frac which is used once and is private into the place where the code is used.
• use std:: for math functions