implemented more array arithmetic operators
added sum and prod arithemetic functions
Merge branch 'master' of https://github.com/pele-python/pele into array_operators
added overall operators tests
Merge branch 'master' of https://github.com/pele-python/pele into array_operators
fixes
removed binary operators and renamed tests
Merge branch 'master' of https://github.com/pele-python/pele into array_operators
Merge pull request #29 from smcantab/array_operators

Array arithmetic operators and other arithmetic functions