Rotation: a diagnostic that opens the prediction window on its own width

Prediction and partiality read one number, the per-image sigma_M, so a sigma_M
that moves takes the integrated reflection population with it and there is no
way to ask which of the two uses carries a downstream difference.
--prediction-mosaicity fixes the width the prediction window opens to while the
partiality keeps using the measured sigma_M, which separates them.

Measured with it on a rotation crystal whose lattice search returns two
different cells a few tens of microns of detector distance apart: widening the
prediction window from the narrower branch's 0.203 deg to the wider branch's
0.272 deg adds 28% more partials and moves the merged statistics by less than
half a percent (I/sigma 6.1 -> 6.2, R_meas 12.7 -> 12.6%, ISa 10.7 -> 10.9);
narrowing the wide branch the other way removes 26% of its partials and
recovers nothing. On a tetragonal reference crystal a 4.7x over-wide window
costs 11%. The prediction window is not where a mosaicity difference turns into
a merged-data difference - the captured-fraction gate and the partiality
weighting downstream absorb a generous window.

Diagnostic only; off by default, so nothing changes unless it is asked for.

common/BraggIntegrationSettings.cpp

@@ -77,6 +77,20 @@ std::optional<float> BraggIntegrationSettings::GetFixedProfileRadius_recipA() co
     return fixed_profile_radius;
 }
 
+BraggIntegrationSettings & BraggIntegrationSettings::ForcedPredictionMosaicity_deg(std::optional<float> input) {
+    if (input) {
+        check_finite("Prediction mosaicity", input.value());
+        check_min("Prediction mosaicity [deg]", input.value(), 0.001);
+        check_max("Prediction mosaicity [deg]", input.value(), 10.0);
+    }
+    forced_prediction_mosaicity_deg = input;
+    return *this;
+}
+
+std::optional<float> BraggIntegrationSettings::GetForcedPredictionMosaicity_deg() const {
+    return forced_prediction_mosaicity_deg;
+}
+
 BraggIntegrationSettings &BraggIntegrationSettings::Integrator(IntegratorMode input) {
     integrator_mode = input;
     return *this;