Proposal to drop `MorphHelper.get`

[asanin-epfl]

Bluepysnap should be an agnostic tool to read Sonata. Currently it forces user to load morphologies via NeuroM which is redundant and too specific. What if user wants MorphIO? Besides we can't do the same with neuron models because there are multiple possible formats. Each format requires its own library. Currently there are 5 formats => 5 additional dependencies just to read some formats. So, I decided not to introduce get in NeuronModelHelper, see PR#123

Here is the link to MorphHelper.get.

My proposal is to drop MorphHelper.get

cc @jdcourcol , @mgeplf , @tomdele

[jdcourcol]

I think this is in close relationship to what @GianlucaFicarelli is doing.