Adds 'start_server.py' as a command-line script

Should be invoked by nxstartserver when it’s properly installed. For
the time being, we need to type ‘python
nexpyro/src/nexpyro/pyro/start_server.py 8801’
Added _readvalue function to NXfile

This allows _readvalue to be customized by the inherited NXFileRemote
class, allowing a one-step remote call.
Refactored nxfileservice

nxfileservice.py now defines the NXFileService class, and
start_server.py is the script that starts the server.
Added _readvalue and read file

‘_readvalue’ calls the nxfileservice getvalue function with both a path
and a key. ‘read file’ loads the whole root tree, as in the parent
NXFile class.
Now loads whole tree into nxfr

‘nxfr’ is no longer the NXFileRemote object, but the NXroot object
returned by the remote tree function.