fix reading of dotprops objects using read.neuron(s)

* and test for this behaviour
Teach read.neuron(s) to accept a format specifier

* and test for this
* so far only used for rda/rds
teach read.neurons how to read a neuronlistfh object

* and check that the result is the same as read.neuronlistfh
restrict read.neurons to filehashRDS backed neuronlistfh objects

* this is the only filehash class in use at the moment
document ability of read.neurons to use neuronlistfh object as input
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)