Create ssc_eqns function that takes the whole vartable as an input

[brtietz]

The existing ssc_eqn function requires setting up a table as an input, which is ok for a small number of inputs, but functions like NatLabRockies/ssc#1412 can have 25+ inputs.

ssc_auto_exec_eqn utilizes the vartable, but requires the function to be stored in the compute module.

Create a new function that calls an ssc equation while passing in the entire vartable.