This change allows users to specify an epoch length of 0, which means
that the user manually has to finish the epochs. A new next_epoch
function is introduced to allow users to manually end epochs.
Addresses GH-348
* All "Broxygen" usages have been replaced in
code, documentation, filenames, etc.
* Sphinx roles/directives like ":bro:see" are now ":zeek:see"
* The "--broxygen" command-line option is now "--zeexygen"
