Let project -G accept increment unit and +n modifier

[PaulWessel]

Description of proposed changes

The distance increment set in project has long been required to be in km, whereas most places in GMT we can specify what unit we want (e.g., 30n for 30 nautical miles or 2d for 2 spherical degrees) but not so in project. We were also unable to specify the number of points, which is pretty handy when you give a start and stop point and want to generate intermediate points. Thus, this PR:

1. Adds the capability of providing the increment with the desired unit appended
2. Adds modifier +n to indicate that the increment is number of points instead (which requires either -C and -E or -Z).
3. Removes +n from -Z but in a backwards compatible way.

All tests pass except an undetected error in ellipses.sh where -Q was not passed so -G10 was interpreted (as it should be) as degrees. The script was updated to use -Q.