Porting Bessel kv function implementation to reduce dependency on tfp

LICENSE

@@ -22,6 +22,24 @@ lost profits, business interruption, or indirect special or consequential
 damages of any kind.
 
 
+Code in bessel.py (fractional-order Bessel K functions) is derived from
+TensorFlow Probability under this license:
+
+Copyright 2020 The TensorFlow Probability Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+
 Code in angle.py and celestial.py is based on LSSTDESC/Coord under this license:
 
 Copyright (c) 2013-2017 LSST Dark Energy Science Collaboration (DESC)