sgo - Simple Geographical Operations (with OSGB36)

• Bumped minimum R required version to 3.5 (sysdata.rda uses workspace format version 3)
• Replaced testthat with tinytest for Unit Testing.
• sgo_coordinates gained a couple of paramaters meant for geodetic coordinates.
• Improved the speed of NGR conversions.
• A little tidy-up of both code and documentation.