Adds FFT magnet ASENS

The FFT magnet has now a fast readback via a dedicated Beckhhoff analog 16bit input terminal.

• Changes the 'magnet' to the ASENS fast readback
• Creates a 'magnet_old' with the old Kepco readback

The attached notebook shows how this works for old and new recorded data sets: 2019-09-05_FFT_MAG_ASENS_test.ipynb