ENH: Added temporal decimation to Morlet analysis

To help minimize memory usage when processing large datasets, added decim
parameter to `single_trial_power` and `cwt`.

This allows me to analyze some datasets where the final result of
`single_trial_power` would not have fit in memory otherwise.  Having `cwt`
perform the decimation before returning the result also sidesteps the issue of
multiple jobs (using joblib) returning large arrays to the parent process that
crashed my Python interpreter.

Also includes a docstring fix for `single_trial_power`.