Applies a power effect to provided epsilons. If a dependent variable
is provided, then only said epsilons affecting said variable will be changed.
Initial estimates for new thetas are 1 if the error
model is proportional, otherwise they are 0.1.
NOTE : If no DVs or epsilons are specified, all epsilons with the same name
will be connected to the same theta. Running the function per DV will give
each epsilon a specific theta.