Allow export (download) of entropy & events

[jameshadfield]

Context
There are frequent requests to access the data behind the entropy panel. Allowing this data to be downloaded similar to how we facilitate SVG & metadata download would be sensible.

Possible solution
The data is stored in redux state (reduxState.entropy.bars) and is kept in sync with any data filtering. It should be simple to use this to write out a TSV linking genome position to entropy & events.