Drivers: add lerp and clamp functions to namespace.

Implementation of lerp without a function requires repeating one of
the arguments, which is not ideal. To avoid that, add a new function
to the driver namespace. In addition, provide a function for clamping
between 0 and 1 to support easy clamped lerp, and a smoothstep function
from GLSL that is somewhat related.

The function implementations are added to a new bl_math module.
As an aside, add the round function and two-argument log to the
pylike expression subset.

Differential Revision: https://developer.blender.org/D8205

source/blender/python/generic/bl_math_py_api.h

@@ -0,0 +1,27 @@
+/*
+ * 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 2
+ * 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, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/**
+ * \file
+ * \ingroup pygen
+ */
+
+#ifndef __BL_MATH_PY_API_H__
+#define __BL_MATH_PY_API_H__
+
+PyMODINIT_FUNC BPyInit_bl_math(void);
+
+#endif /* __BL_MATH_PY_API_H__ */