Allow generation of imputation values on command

[samFarrellDay]

Actually storing the latest imputation values can take up a lot of memory. We have all the information we need to generate imputation values when complete_data() is called, why not give the user that option.

Easiest way would be to change save_all_iterations to an int parameter. 0=generate on demand, 1 = save latest iteration, 2=save all iterations.