ENH: Allow ARMA(0,0) and ARIMA(0,d,0) models with constant/trend models to be estimated

* Allows ARMA and ARIMA with only exogenous or constant terms
* Changed the IC to match the definition in OLS as well as most other sources (see, e.g. 23.2 of http://gretl.sourceforge.net/gretl-help/gretl-guide.pdf).  The currently produces a large number of failures.  Would be useful to check source for baseline results **Note**: All differ by a term which has no impact on model selection.
* Still needs test for pure exogenous models

Does **_not_** try to implement "parameterless" models.