Version 2.1: * Added new function `AccelStructure::getF0()`, similar to `::getOmega()` . * Constructors `AccelStructure_paramSet1`, `AccelStructure_paramSet2`, `AccelStructure_paramSet2_noPsi` now have optional argument `f0_scaleto`. * Flags for enabling/disabling gradient limits - default is enabled. Simply exposed as public variables in the `AccelStructure` objects: `uselimit_E = true; bool uselimit_Sc = true; bool uselimit_PC = true; bool uselimit_dT = true;`