central_meridian instead of projection_origin in grid mapping

This PR removes the convoluted history from tmean_1910_1910.nc and adds a new lightweight file that has longitude_of_projection_origin and scale_factor_at_projection_origin in place of longitude_of_central_meridian and scale_factor_at_central_merdidian as shown in example 5.10 on http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/ch05s06.html