Rejig weight and conversions for modcc (#800)

Address inconsistencies in unit conversions between current updates, initialization, ionic currents etc.

Fixes issues #793 and #798.

* Correct units in comments for ion state iX_ to a current density.
* Remove conversion factor for NMODL current densities [mA/cm²] to Arbor current densities [A/m²] from layout weight: from the point of view of Arbor, mechanisms now write in units that Arbor uses natively.
* Remove weight_ as a magic modcc variable; applying weight_ is now performed directly by code generated by printers.
* Change IndexedVariable interface: remove redundant index_name; remove 'op'. Whether external data is accumulated or assigned is now a property of the corresponding sourceKind.
* Conversion factors for NMODL <-> Arbor are collated in printerutils.cpp, determined by the external data sourceKind.
* Permit direct assignment of external data, as opposed to a weighted sum. This behaviour will be required for reversal potential handling.
* `current_` and `conducitivity_` get a different sourceKind in point mechanisms, allowing us to distinguish current contributions from current density contributions (and applying correct unit scaling accordingly.)
* Add unit test for correct initialization and scaling of ionic currents, together with two corresponding unit test mechanisms.