rugnux: the gain of a stills file that states none is read off its single-photon peak where the background resolves photons

A pedestal-subtracted JUNGFRAU background is mostly empty pixels; the variance per unit mean leaves
out the zeros and read 6.7 keV per photon on a 12 keV run in keV, 2.4 on a 4.6 keV one, so such
files needed --gain. MeasureGain now also histograms the sampled pixel values and takes the gain from
the single-photon peak (background_gain::PhotonPeak): the highest bin past the zero peak's descent,
standing 5 sigma above the valley before it, narrow (below half height a quarter of its value away),
and repeated by a two-photon peak near twice its value. Where there is no such pair the variance per
unit mean decides as before. Float frames are read for the measurement at a sixteenth of their unit,
so a photon of a few keV is resolved rather than rounded to whole keV.

Measured: float32 keV JUNGFRAU files 11.96 (photon energy 12.04 keV) and 4.53 (4.57 keV); the int16
photon copies show no peak past the first bin and keep the 0.55 variance answer, i.e. stay in
photons; the SACLA MPCCD and PAL Rayonix files show humps, not photon peaks, and measure as before.
A gain the file states or --gain gives still wins. Rotation is untouched (the pre-scan's CCD gain
uses only the variance estimator).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu

reader/JFJochXFELReader.cpp

@@ -775,6 +775,16 @@ void JFJochXFELReader::ApplyGain(float adu_per_photon) {
     Place();
 }
 
+void JFJochXFELReader::ClearGain() {
+    for (auto &p: geom_.panels)
+        p.adu_per_photon.reset();
+    Place();
+}
+
+bool JFJochXFELReader::StoredFloat() const {
+    return layout_.stored_float || !layout_.plain_type;
+}
+
 bool JFJochXFELReader::HasPhotonEnergy() const {
     return photon_energy_eV_ > 0;
 }