Update license text

LICENSE

@@ -1,16 +1,16 @@
+libconviqt -- 4pi beam convolution library
 
-libconviqt
+Copyright (C) 2014-2019 Gary Prezeau and Reijo Keskitalo
 
-Copyright (c) 2015-2019, by the California Institute of Technology.
-ALL RIGHTS RESERVED.
+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 3 of the License, or
+(at your option) any later version.
 
-United States Government Sponsorship acknowledged.
+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.
 
-Any commercial use must be negotiated with the Office of Technology
-Transfer at the California Institute of Technology.
-
-This software may be subject to U.S. export control laws. By accepting this
-software, the user agrees to comply with all applicable U.S. export laws and
-regulations. User has the responsibility to obtain export licenses, or other
-export authority as may be required before exporting such information to foreign
-countries or providing access to foreign persons.
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.