Update -g synopsis, syntax, usage and docs

[PaulWessel]

The -g syntax was a bit awkward (had a separate -qa to change a bool value). Now we use a modifier (+a) like elsewhere. Also, we now set an alternative z-coordinate column via +ccol like elsewhere [2]. Also fixed wording and minor mistakes in the RST and synopsis.

[maxrjones]

I find the new syntax more confusing than before (in particular the relation between z directive and +c modifier). I think this would be more similar to the syntax in other common options:
-gx|y|d|X|Y|D|z[col/]gap[+a][+n|p]

[PaulWessel]

Let's discuss this. I see +ccol is used in several places:

Common options:

-qi[~]<rangevalues>,...+c<col>
-qo[~]<rangevalues>,...+c<col> 
-wy|a|w|d|h|m|s|c<period>[/<phase>][+c<col>]

gmtinfo:
[-T<dz>[+c<col>]]

gmtselect:
-Z<min>/<max>[+c<col>][+i][+a]

In that context, this looks similar to me:

-gx|y|z|d|X|Y|D<gap>[<unit>][+a][+c<col>][+n|p

Adding the slash if a zcol is specified means more complications in doing a long-option I think. PS. I need to read a thesis and attend a defense and then a Dr visit so will be very intermittent here today.

[maxrjones]

Let's discuss this. I see +ccol is used in several places:

Common options:

-qi[~]<rangevalues>,...+c<col>
-qo[~]<rangevalues>,...+c<col> 
-wy|a|w|d|h|m|s|c<period>[/<phase>][+c<col>]

gmtinfo:
[-T<dz>[+c<col>]]

gmtselect:
-Z<min>/<max>[+c<col>][+i][+a]

In that context, this looks similar to me:

-gx|y|z|d|X|Y|D<gap>[<unit>][+a][+c<col>][+n|p

Adding the slash if a zcol is specified means more complications in doing a long-option I think. PS. I need to read a thesis and attend a defense and then a Dr visit so will be very intermittent here today.

OK, I will instead just make a couple small wording suggestions for the new syntax.

[maxrjones]

Looks good after the two suggestions removing the extra | in the synopsis are resolved.