add Cephes README and License

gtsam/3rdparty/cephes/LICENSE.txt

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+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

gtsam/3rdparty/cephes/README.md

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+# README
+
+This is a vendored version of the Cephes Mathematical Library. The source code can be found on [netlib.org](https://www.netlib.org/cephes/).
+
+The software is provided with an [MIT License](https://smath.com/en-US/view/CephesMathLibrary/license).
+
+## Original Readme
+
+Some software in this archive may be from the book _Methods and
+Programs for Mathematical Functions_ (Prentice-Hall or Simon & Schuster
+International, 1989) or from the Cephes Mathematical Library, a
+commercial product. In either event, it is copyrighted by the author.
+What you see here may be used freely but it comes with no support or
+guarantee.
+
+The two known misprints in the book are repaired here in the
+source listings for the gamma function and the incomplete beta
+integral.
+
+
+Stephen L. Moshier
+[email protected]