Added a kappa dependence to ptau in the DBSTSS.nb script

I've added support for changing kappa when calculating ptau, which
allows me to see more of a range of what the input currents should be.
First time inclusion of TimeConstantCalculator.nb

TimeConstantCalculator.nb is a script that takes input from the
simulation platform of a waveform and calculates the time constant of
the exponential curve.  It doesn't yet fit the parameters fully
accurately, but it does grab the data and plot the files.