read.neuronlistfh now adds the location of local file to neuronlistfh object

* this will be necessary for the synchronisation routines
Add write.neuronlistfh function
test that write.neuronlistfh behaves as saveRDS
Teach write.neuronlistfh to remove any file attribute
doc improvements
Don't update neuronlistfh db@dir if it points to a valid path

* this means that it remains possible to put the neuronlistfh and filehash
  database directory in different relative locations
tests for modifications to read.neuronlistfh
update @family directives
change as.neuronlistfh.neuronlist argument dir->dbdir

* to emphasise that this path is indeed the database directory
fix references to renamed (db)dir argument of as.neuronlistfh
Add remotesync generic and implementations for neuronlistfh and paths
remotesync: only update neuronlistfh object if requested
fix construction of path to remote neuronlistfh object

* and add basic test
* and document limitation for update.object
fix incorrect setting of file attribute when downloading in read.neuronlistfh

* should be set to local cached copy, not remote
fix bugs in downloading missing files (and add tests)
fix bug in deleting extra files (and add tests)
Merge branch 'feature/remotesync'

* feature/remotesync:
  fix bug in deleting extra files (and add tests)
  fix bugs in downloading missing files (and add tests)
  fix incorrect setting of file attribute when downloading in read.neuronlistfh
  fix construction of path to remote neuronlistfh object
  remotesync: only update neuronlistfh object if requested
  Add remotesync generic and implementations for neuronlistfh and paths
  fix references to renamed (db)dir argument of as.neuronlistfh
  change as.neuronlistfh.neuronlist argument dir->dbdir
  update @family directives
  tests for modifications to read.neuronlistfh
  Don't update neuronlistfh db@dir if it points to a valid path
  doc improvements
  Teach write.neuronlistfh to remove any file attribute
  test that write.neuronlistfh behaves as saveRDS
  Add write.neuronlistfh function
  read.neuronlistfh now adds the location of local file to neuronlistfh object