Add flux/mag conversion code

No python bindings because astropy handles this directly.

Lifted out of afw PhotoCalib.

include/lsst/utils/Magnitude.h

@@ -0,0 +1,56 @@
+// -*- LSST-C++ -*-
+/*
+ * This file is part of utils.
+ *
+ * Developed for the LSST Data Management System.
+ * This product includes software developed by the LSST Project
+ * (https://www.lsst.org).
+ * See the COPYRIGHT file at the top-level directory of this distribution
+ * for details of code ownership.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * @brief Utilities for converting between flux and magnitude in C++.
+ *
+ * Use `astropy.units` `ABmag` and `nJy` for converstions in python:
+ * @code{.python}
+ *   import astropy.units as u
+ *   mag = (flux*u.nJy).to_value(u.ABmag)
+ *   flux = (mag*u.ABmag).to_value(u.nJy)
+ * @endcode
+ */
+
+#ifndef LSST_UTILS_MAGNITUDE_H
+#define LSST_UTILS_MAGNITUDE_H
+
+#include <cmath>
+
+namespace lsst {
+namespace utils {
+
+/// The Oke & Gunn (1983) AB magnitude reference flux, in nJy (often approximated as 3631.0).
+const double referenceFlux = 1e23 * pow(10, (48.6 / -2.5)) * 1e9;
+
+/// Convert a flux in nanojansky to AB magnitude.
+double nanojanskyToABMagnitude(double flux);
+
+/// Convert an AB magnitude to a flux in nanojansky.
+double ABMagnitudeToNanojansky(double magnitude);
+
+}  // namespace utils
+}  // namespace lsst
+
+#endif  // LSST_UTILS_MAGNITUDE_H

src/Magnitude.cc

@@ -0,0 +1,35 @@
+// -*- LSST-C++ -*-
+/*
+ * This file is part of utils.
+ *
+ * Developed for the LSST Data Management System.
+ * This product includes software developed by the LSST Project
+ * (https://www.lsst.org).
+ * See the COPYRIGHT file at the top-level directory of this distribution
+ * for details of code ownership.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "lsst/utils/Magnitude.h"
+
+namespace lsst {
+namespace utils {
+
+double nanojanskyToABMagnitude(double flux) { return -2.5 * log10(flux / referenceFlux); }
+
+double ABMagnitudeToNanojansky(double magnitude) { return pow(10, magnitude / -2.5) * referenceFlux; }
+
+}  // namespace utils
+}  // namespace lsst