Cumulative changes from gwpy-plot testing

Created by: areeda

Sorry for the "merge" commits rebase failed before each one. Annotation for changed files:

bin/gwpy-plot

• move test for no DISPLAY to top for matplotlib.use()

gwpy/cli/cliproduct.py

• :rtype annotation added to log() docs

gwpy/cli/qtransform.py

• def fformat(x): # float format failed if passed a str arg

gwpy/cli/spectrogram.py

• Deal with zeroes in resulting spectrogram and log color scale