Refactor Function management and add fitting functions

[sarakurkowska]

Add different sum of exponential functions, according to:
1 \ Hardiansyah D, Riana A, Kletting P, Zaid NRR, Eiber M, Pawiro SA, Beer AJ, Glatting G. A population-based method to determine the time-integrated activity in molecular radiotherapy. EJNMMI Phys. 2021 Dec 14;8(1):82. doi: 10.1186/s40658-021-00427-x. PMID: 34905131; PMCID: PMC8671591.

2\ https://aapm.onlinelibrary.wiley.com/doi/full/10.1118/1.2794176

3\ peter jaskcon triexponential fitting