time frequency code now allows for alternative normalizations

named argument normfun in avg_time_frequency accepts a 1-parameter
function that takes a list of spectrum dataframes (one per trial)
and returns a similar list with entries normalized

functions norm_by_trial and norm_by_mean are factory functions that
take a time range in the form of a tuple and return such 1-parameter
functions; the former normalizes within a trial, the latter
within the grand mean across trials