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/intern/bpy_interface.c

@@ -68,6 +68,7 @@
 /* inittab initialization functions */
 #include "../bmesh/bmesh_py_api.h"
 #include "../generic/bgl.h"
+#include "../generic/bl_math_py_api.h"
 #include "../generic/blf_py_api.h"
 #include "../generic/idprop_py_api.h"
 #include "../generic/imbuf_py_api.h"
@@ -228,6 +229,7 @@ static struct _inittab bpy_internal_modules[] = {
     {"_bpy_path", BPyInit__bpy_path},
     {"bgl", BPyInit_bgl},
     {"blf", BPyInit_blf},
+    {"bl_math", BPyInit_bl_math},
     {"imbuf", BPyInit_imbuf},
     {"bmesh", BPyInit_bmesh},
 #if 0