Pvapy

- added build support for python3
- added build support for EPICS7 releases

- improved support for channel monitors: no monitor startup thread results in
faster initial connections; monitors connect automatically when channels
come online
- fixed support for older EPICS v4 releases (4.4, 4.5 and 4.6)
- added build support for numpy included with boost releases 1.63.0 and later

- added new Channel monitor() method that can be used instead of
subscribe()/startMonitor() sequence in case when there is only one
subscriber
- enhanced PvaServer functionality: single server instance can serve multiple
channels; channels can be added and removed dynamically; added (optional)
callback for channel writes

- added initial version of PvaServer class: PvObject exposed via instance of
this class can be retrieved and manipulated using standard PVA command line
tools and APIs
- fixed boolean array conversion to python list
- improved support for builds relying on custom boost installation