Implement float support to FOR IN RANGE loops (fixes #1850)

FOR IN RANGE loop works now even with float arguments.
Parameters for `FOR IN RANGE` are processed in `src/robot/running/keywords.py` (it was your suggestion to move the parameter evaluation there), so we should never receive in `frange` function anything else besides int and float. 
What should be probably done is to add one more testcase in `atest\robot\running\for.robot` to verify this feature. I already did some tests using scientific notation for the parameters of `FOR IN RANGE` and it works.