Cache result from yamlCamera.makeCamera

Constructing a camera from YAML can take a long time
(10 seconds for imsim) but the resulting object is
immutable.  Caching can significantly speed up code
that creates multiple cameras (in particular gen2 to 3
conversion code).

python/lsst/obs/base/yamlCamera.py

@@ -21,6 +21,7 @@
 
 import yaml
 
+from functools import lru_cache
 import numpy as np
 import lsst.afw.cameraGeom as cameraGeom
 import lsst.geom as geom
@@ -31,6 +32,7 @@
 __all__ = ["makeCamera"]
 
 
+@lru_cache(maxsize=20)
 def makeCamera(cameraFile):
     """An imaging camera (e.g. the LSST 3Gpix camera)