Wrong modification of the analog scales in the C3D file format

What steps will reproduce the problem?
1. Open a C3D file format in BTK (Mokka 0.6 / BTK 0.2 in Matlab/Python) with 
negative analog scale
2. Save the loaded acquisition as new C3D file
3. Load the new file
4. Check the analog scales and the general scale (visible in the metadata view)

What is the expected output? What do you see instead?
The analog scales are multiplied by 10000 and the general scale is set to 1e-5 
but these scale factors should not be modified.

Note: The saved data are still good but could potentially create aliasing if 
you add other analog channels.

Please use labels and text to provide additional information.
The issue is due to the method used for the computation of the general analog 
scale. Indeed, instead of checking for small values (less than 1e-5), the test 
checks for negative values.

Original issue reported on code.google.com by arnaud.barre on 8 Apr 2013 at 7:51

ZENOUDA / b-tk

Wrong modification of the analog scales in the C3D file format #57