Add integral_of_R, integral_of_R1, integral_of_R2, ... functions

src/language/modules/MathFunctionRegisterForVh.cpp

...language/utils/EmbeddedIDiscreteFunctionMathFunctions.cpp

...language/utils/EmbeddedIDiscreteFunctionMathFunctions.hpp

src/mesh/ItemValueUtils.hpp

src/scheme/DiscreteFunctionP0.hpp

src/utils/ArrayUtils.hpp

src/utils/PugsUtils.hpp