Remove inverseTransValid flag from NumericAggState

This is not required as if we return NULL from the inverse transition function the aggregate state will be initialised again in retreat_windowaggregate(). The forward transition function will always be called again before we ever call the inverse transition function.
Remove state == NULL checks in strict inverse transition functions.

These are not required as the functions will never be called with a NULL state.
Also added comments to numeric_avg_accum_inv() and int8_avg_accum_inv() to state
that these functions must be declared as strict.
Merge commit 'eafa723' into invtrans
Try to improve on patch to add the average transitions per row to explain output

This moves the original code out into a helper function to match what the other code
does. I've also added a check so that the transitions per row does not show if there
are no aggregate functions in the window clause.