Prevent warning about ambiguous order of operations in BumpGrid.cc. Add
parenthesis to maintain the behavior of the code as per C++ operator
precedence and associativity rules. This warning is emitted by gcc
5.1.0 and prevents compilation with scons. (This is the tweak
originally intended by commit 8c0d064ab8d5f89654483308c764dc19e83a818b.)