Freature branch for adding the on-the-fly sampling rate conversion feature

A crude version doing the nearest-neighbor interpolation. Sampling rate
optional key-value pair 'fs' in the block function now overrides 'fs'
read from a wav file.
Added a sanity check to vect2cell
On-the-fly linear and Lagrange interpolation added to Playrec

Works for any sampling rate ratios. Upsampling (expecially by large
factor) works nice even with linear interpolation. Downsampling is bad
with both because the aliasing is not removed.
Fix for bug reported at http://savannah.gnu.org/bugs/?42278
Added check to memory allocation routines in ltfat_mex_template_helper.h
ltfatstart now omits oct/mex subdirectories when run form Matlab/Octave

mex/postpad is no longer added to path on Octave.
Trivial change in ChangeLog
Antialiasing filter in resampling reoutine in progress
Addition of energy spectral density function.
Added enspecden to Contents file of Fourier section.
Blockproc demos close panels when initialization fails
Working (although probably fragile) resampling in real-time including anti-aliasing filtering.
Fixed demo_auditoryfilterbank (again) and filterbankresponse
FIR filters are now not allowed to be used with fractional subsampling
An Octave compatibility fix in comp_iuwfbt.m
Adjusting block lengths according to sampling rate in blockproc demos.
More tweaking on ltfatresample

Output overflow no longer causes access to uninitialized memory.
Added a sanity check to vect2cell
Added tests for ltfatresample and fixed some memory errors
Closing #100 FEATURE: Add on-the-fly sample rate conversion to the blockproc framework