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