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
clean up cython lbfgs optimizer

by declaring events (cpdef) we can attach it to the c++ (cdef) class.
Then we don't need to separate to functionality into python plus c++ and pure
c++
update modified fire and gradient optimizer

they are both simplified now that events is part of the unified
framework
lbfgs now derives from the cdef gradient optimizer
add H0 back in to the LBFGS result object
fixes
introduced periodic 2D

added 2D cases to distance.h and modified pairwise potential and
pairwise_ilist potential to work for arbitrary number of dimensions
added 2D WCA and HS_WCA potential + performance improvements to WCA
Merge branch 'master' of https://github.com/pele-python/pele into periodic2D
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
Merge branch 'master' of https://github.com/pele-python/pele into periodic2D
update c++ example potential

There were several changes made to the c++ backed that were not
reflected in the example
fix to com of harmonic
bug fix harmonic com