Cf grids

This PR changes the default value of the transform and projection formatoptions to cf. If the projection is encoded in the dataset following the cf-conventions (see http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#appendix-grid-mappings), the projection is automatically decoded and used. If no projection information can be found, the standard cyl projection is used.

Furthermore, transform and projection also allow the 'rotated' value to use a rotated pole.

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes