Option to choose exact minimum resolution

[danieledler]

Now the option is limited 2^x but should be generalised to y*2^x, so that we could have for example exactly 10" resolution minimum. Now constrained to:

deg, min, sec
1, 60, 3600
1/2, 30, 1800
1/4, 15, 900
1/8, 7.5, 450
1/16, 3.75, 225
1/32, 1.875, 112.5
1/64, 0.9375, 56.25
1/128, 0.46875, 28.125,
1/256, 0.234375, 14.0625,
1/512 0.1171875, 7.03125