MAINT: demos: remove demos that don't add anything, add comment on C API.
MAINT: demos: update benchmarks.py and dwt_signal_decomposition.py
MAINT: demo: update dwt_swt_show_coeffs.py and plot_wavelets.py
MAINT: demo: update swt2.py demo, add figure and axis titles.
MAINT: demo: remove user_filter_bank.py, that's covered in the docs already.
MAINT: demo: update WaveletPacket and WaveletPacket2D demos.
MAINT: demo: remove wp_tree.py, not useful.
MAINT: demo: update waveinfo.py
MAINT: move ecg data into a .npy data file.
Merge branch 'demo-update' into master.

- Updates all files in the demo dir to currently standard idioms.
- Removes not-so-useful files.
- Adds some comments and figure titles to make things more understandable.