added "negative" as option to noise scaling

the noise scaling function only had "std" and "mad" as option to
calculate rms. "negative" is now added as an option similar to other
tasks were rms is used
Noise scaling is "cleaned up"

The code is shortened and cleaned up. Rather than doing the rms
calculation in the script, the GetRMS function in functions.py is used
to make it more standardised with other tasks
GetRMS works for 2D input

the GetRMS function in functions.py should work with all default python
libraries and deal properly with NaNs. Also a line is added to checker
whether the input is 2D and anticipate to that.