add sutton chen potential
Merge branch 'master' of github.com:pele-python/pele
add a sutton chen system class to playground
minor updates to sutton chen potential
database checks if properties are the same

if they're not the same (or old==new raises an error) a warning message
is issued.
xy model now stores quenched disorder in database
change default quench tolerance back to 1e-6

It was 1e-4 for a few weeks, but I was too often getting transition
states to be recognized as minima for LJ13.
use defaultdict in the rates calculation setup
setup pyro4 worker to run parallel basinhopping jobs

the basinhopping worker passes new minimna to ConnectServer, so both
basinhopping jobs and connect jobs can be run simultaneously.

Update the example and move it to folder parallel_pele/
moved headers to subdirectory to avoid naming conflicts
removed restrict (not c++ standard)
added helper functions to array
replace assert(_size == rhs.size()) with exceptions

also fix the depends in setup.py
add a script that will start the pyro jobs and workers

for parallel basinhopping and connect jobs.

The only information needed about the system is in the System Class.
The system class is obtained from a function create_system() which is
imported from a file or module passed to the script
add flag `overwrite=True` to database.add_property

BaseSystem checks for params.database.overwrite_properties
change default quench tolerance to 1e-5
update to xy_model
updates to parallel_pele.py script
added fix for missing round in visual studio
moved norm and dot to array.h, added missing inline and simplified naming