EXP: Exponential time dependent value

## 4.4 `EXP`: Exponential time dependent value

### 4.4.1 Syntax

`EXP` args
`EXP` iv pv td1 tau1 td2 tau2 period

### 4.4.2 Purpose

The component value is an exponential function of time.

### 4.4.3 Comments

For voltage and current sources, this is the same as the Spice `EXP` function, with some extensions.
The shape of the waveform is described by the following algorithm:

    ev = _iv;
    for (reltime=time; reltime>=0; reltime-=_period){
      if (reltime > _td1){
        ev += (_pv - _iv)
              * (1. - Exp(-(reltime-_td1)/_tau1));
      }
      if (reltime > _td2){
        ev += (_iv - _pv)
              * (1. - Exp(-(reltime-_td2)/_tau2));
      }
    }

### 4.4.4 Parameters

`IV` = x
  Initial value. (required)
`PV` = x
  Pulsed value. (required)
`TD1` = x
  Rise time delay. (Default = 0.)
`TAU1` = x
  Rise time constant. (Default = 0.)
`TD2` = x
  Fall time delay. (Default = 0.)
`TAU2` = x
  Fall time constant. (Default = 0.)
`Period` = x
  Repeat period. (Default = infinity.)