[add] print method for neuronlistz

[dokato]

Addresses #466 .

I simplified so it doesn't call expensive contents.classes <- sapply(x, firstclass, USE.NAMES = F).

Artefact, we don't print the information about neuron types.

> nzmv
'neuronlistz' containing 3000 neurons objects and 'data.frame' with 1 vars [998 kB]

Benchmark on 3000 neurons:

# old
> system.time(print(nzmv))
'neuronlistz' containing 3000 'neuprintneuron' objects and 'data.frame' with 1 vars [998 kB]
   user  system elapsed 
 16.702   1.805  19.000 
# new
> system.time(print(nzmv))
'neuronlistz' containing 3000 neurons objects and 'data.frame' with 1 vars [998 kB]
   user  system elapsed 
  0.000   0.000   0.001 

Alternatively, we can add another rds that stores that info in zip? WDYT @jefferis ?

[coveralls]

Coverage increased (+0.04%) to 81.425% when pulling 0bc9ef3 on dokato:printneuronlistz into 88507a8 on natverse:master.

[jefferis]

Thanks @dokato. This is good, but can you add the path to the zip file and its size on disk to the displayed information and make it clear that the currently printed size is just for the portion in memory?

[dokato]

Is message like this okay?

'neuronlistz' containing 3000 neurons objects and 'data.frame' with 1 vars.
Neurons loaded on demand from 'testz.zip' [93.1 MB]. Currently in memory: 998 kB.

[jefferis]

Thanks! How about:

'neuronlistz' containing 3000 neurons objects and 'data.frame' with 1 vars [998 kB in RAM].
Loaded on demand from '/x/y/z/testz.zip' [93.1 MB on disk].

[dokato]

Sure, done!

[jefferis]

Thanks again @dokato!