New pp rule to calculate forecast period.

This is a replacement for #503

I have a cube loaded from a netcdf file. The cube is is a time mean and has a scalar time coordinate with bounds. The cube also has a forecast_reference_time but does not have a forecast period. When saved to PP and then loaded again the time coordinate is corrupted and the forecast_reference_time is lost.

This PR adds a new PP save rule to handle the case of a missing forecast period. It reconstructs the forecast period from the upper time bound and the forecast_reference_time and assigns it to lbft.

Please see the comments in #503