Activity plot with unix pipe interface

Graphactivity.py creates a pipe, /tmp/activity, which waits for data to
be written from another program.  Data has the form: one time step per
line, with each time step containing a series of token-count pairs, e.g.

token1 count1 token2 count2 …

An evolutionary simulation should establish what tokens are being
counted, then output these lines to /tmp/activity.

Usage:
First start graphactivity.py
Second start the evolutionary program that will write data to
/tmp/activity (opened as a file for writing).