Update install script to work on windows mantid installation

install_wfm_stitching.py

@@ -69,6 +69,14 @@ if __name__ == "__main__":
     parser.add_argument('-u', '--uninstall', action='store_true', default=False,  help='uninstall any existing wfm-stitching package')
     args = parser.parse_args()
     interpreter = sys.executable
+    opt1 = sys.executable
+    if os.path.exists(opt1):
+        interpreter = opt1
+    else:  
+        opt2 = os.path.join(os.path.dirname(sys.executable), 'python')
+        interpreter = opt2 
+    if not os.path.exists(interpreter):
+        raise RuntimeError("Cannot find interpreter {0} or {1}".format(opt1, opt2))
     if sys.version_info < (3, 5):
         raise RuntimeError("Python version too old. Needs to be >= 3.5, but found {}".format(repr(sys.version_info)))
     if args.uninstall: