ThibHlln/eflowcalc: Version with almost all existing SFCs

Semantics Enhancements:

• adds 142 streamflow characteristics of the 171 inventoried by Olden and Poff (2003)

Functionality Enhancements:

• adds possibility to select specific years where to calculate the SFCs
• adds gathering functions for each flow category (magnitude, frequency, timing, duration, rate_change) and for all SFCs (everything)

Algorithm Enhancements:

• adds single-sourcing package version
• changes default array orientation
• changes the degrees of freedom to n-1 in calculation of std (breaking backward compatibility)