Steady state values used directly in current equation in IT_huguenard.mod

[JustasB]

In the original version, the current equation uses the steady state value of one of the gating variables directly:

ica = gcabar * h * m_inf * m_inf * (v-carev)

This may have been contributing to small differences between LEMS -neuron and original version.

In modified version, I changed this to:

ica = gcabar * h * m * m * (v-carev)

Added m state variable, and added the following:

m' = (m_inf - m) / tau_m
tau_m = 1.0