Ensure default values in cell params geom and biophysics

[pgleeson]

cm is missing here for example: https://github.com/Neurosim-lab/netpyne/blob/master/examples/HybridTut/HybridTut.py#L36

Ideally a default value should be added for each once the params are loaded in so they can always be accessed later with secs.soma.geom.cm etc.

[salvadord]

• Ensure all of gnabar/gkbar/ena etc. set on cellRule['secs']['soma']['mechs']['hh'], or use defaults

[salvadord]

add in cell.createNEURONObj() function by iterating over all params of the mech (use utils.mechVarList) and adding missing params with Neuron default value