Load balancing roadmap

[streeve]

• Create a base LoadBalancer class with the particle workload that exists in SparseDimPartitioner
• Inherit from LoadBalancer and overload a couple methods for the SparseLoadBalancer class
• Rename existing class LoadBalancerALL (or similar) and inherit from LoadBalancer

Maybe we need to discuss again what really belongs in the global grid vs partitioner vs load balancer, but the current steps may be enough

[streeve]

Draft in #541