Fixed a bug in the empi2tf() function that caused TF
maps to be displayed incorrectly for channels other than the first one
(only applies to signals with more than one channel).
clear.cache() function. Before deleting files from
the cache, it displays a list of them and asks the user for permission
to delete them.
empi.execute() function. Additional validation has
been added to ensure that list items have the required names
(signal and sampling.rate).
empi.install() function. Added error handling for
download.file() function.