Add postamble option to allow padding of concatenated file

EVS and IVAS decoder always output signals which are frame-length aligned (20ms) and may be shorter or larger than the input signal. If the input file is not 20ms-aligned, the output files will differ in length from the input. As all other processing retains file length (except for filter, but there is already compensation in place), this could lead to a mismatch in signal length between conditions which may be a problem depending on the listening test software.

Proposal to work around that is:

• after file concatenation, append a "postamble" of 1 frame with +/-4 low-level noise to ensure that the output file is always larger than the input
• when splitting the file during postprocessing, throw away the excess samples