Improve performance #153
Description
As noted in #146 iterating over a large number of CellGeometry objects and corresponding forward-model predictions on a single physical machine may take a long time. We could with relative ease add numba.jit style decorators (https://numba.pydata.org) to a number of functions and classes in lfpykit.lfpcalc, lfpykit.models and lfpykit.eegmegcalc as we are mostly relying on pure numpy operations.