Updated Variable Units and Time Step

After recitation on 9/24, used new time step vector for simulation and
removed exponentials on constants for easier manipulation, plan to plot
in mV, mS, etc... instead of base units of V, S, etc.
Added Current Equations
Declared Input Current

Forgot to declare input current as a parameter, allows to change input
current magnitude. Plan to make a vector in consideration of step
function in problem 2.
Added primary loop

Iterating through all time steps and calculating variables in each
iteration using time step and euler's method.