BUG: fixes energy conservation problems for STFT with tight frame

This pull request fixes a bug in the computation of the signal energy in the time-frequency domain. With this, energy in TF and time domain are the same for an STFT with a tight frame (test are now with decimal=6, before it was decimal=2). The problem was in the reduction and reconstruction of TF coefficients for a real-valued signal. 
why this change?