Add proxy management, managing resources in a clever way

With the ProxyMixin.reinstate_proxy method, it becomes possible to do some data management in scenarios where there is a lot of data / little memory.

Proposal

• Can use a apply_ufunc method in BaseDataGroup, allowing to recursively apply the reinstate_proxy method to certain leaves
• Add a "Proxy Management" method the DataManager
  • Needs to be called explicitly
  • Can use the resource module to determine whether memory is low
  • If a threshold is passed, can reinstate some proxies
  • Ideally, reinstate those that were not used in a long time
    • for that, could extend .data property of proxy mixin to store a timestamp
• Alternatively, have something like a global singular proxy registry, a ProxyManager, that takes care of all the stuff and is only invoked by the DataManager...