new constants used in some trigonometric functions
suppressing integer benchmarks for some trigonometric functors
modifying/adding exhaustive test for some scalar/simd invtrig functions
unit tests for the desired properties of pio_2lo and pio_4lo
acceleration and improvement of acos/asin/atan (mainly for float)

- using factorised constant
- changing algorithm for acos
- simplifying and improving atan (atan is now bost faster and at 0.5ulp (1ulp before)