[HazenBabcock]

Running the diagnostics one after another stops on a plot window.
frc/frc_calc2d.py has show_plot = True by default and
diagnostics/frc/analyze_data.py calls it without saying otherwise, once per
test directory. Its command line already has --no-plot; only the
programmatic call was missing the equivalent.

Two changes, because the immediate cause and the general one are different.

The call site

diagnostics/frc/analyze_data.py now passes show_plot = False. An analyze
step that loops over directories should never wait for someone to close a
window.

The general case

STORM_ANALYSIS_HEADLESS in the environment now selects a non-interactive
matplotlib backend, set in __init__.py before pyplot is imported. That
makes every pyplot.show() in the package a no-op — all 21 of them — without
touching a single call site.

Worth having because the flags controlling this are not consistent:

module	flag	note
frc/frc_calc2d.py	show_plot	defaults to True
multi_plane/plane_weighting.py	no_plots	inverted sense
rcc/rcc_drift_correction.py	make_plots
micrometry/micrometry.py	show, save_as

Four flags, three spellings, one inverted. Anyone adding a diagnostic has to
know which applies, and getting it wrong produces a hung script rather than an
error.

Figures written to disk are unaffected

savefig() works the same under Agg. Checked rather than assumed: directly,
and by re-running the multicolor diagnostic headless and confirming it still
produces its three mapping images.

Verified that the default is unchanged (tkagg here), that with the variable
set the backend is Agg and show() returns instead of blocking, and that
the frc diagnostic's analyze step drops from 9s to 2s because it no longer
builds the figure.

Also adds isHeadless(), for code that would rather skip building a figure it
cannot display.

The diagnostics README says how to use it, and — while there — that
multiplane/configure.py needs --psf-model, which is the one thing in that
directory you cannot work out from the README.

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Full test suite passes, 280 tests. All 17 diagnostics were then run end to
end with this branch, 68 steps, no failures; multiplane was run three times,
once per PSF model.