Version 2.1

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;