Add suitable shorthands for creating month and day labels

[PaulWessel]

When building a categorical CPT for months or weekdays, we can now have GMT supply those labels based on current default settings for time and language by using +cM or +cD, respectively. Available in makecpt and grd2cpt. Closes #5189 and we may merge into master after 6.2.0 release.

Examples of use:

gmt makecpt -Crainbow -T1/12/1 -F+cM

1	201.88/0/255	L	;January
2	95.625/0/255	L	;February
3	0/10.625/255	L	;March
4	0/116.88/255	L	;April
5	0/223.12/255	L	;May
6	0/255/180.63	L	;June
7	0/255/74.375	L	;July
8	31.875/255/0	L	;August
9	138.12/255/0	L	;September
10	244.37/255/0	L	;October
11	255/159.38/0	L	;November
12	255/53.125/0	B	;December
N	128

gmt makecpt -Crainbow -T1/7/1 -F+cD --FORMAT_TIME_PRIMARY_MAP=Abbrev --GMT_LANGUAGE=es

1	163.93/0/255	L	;LUN
2	0/18.214/255	L	;MAR
3	0/200.36/255	L	;MIE
4	0/255/127.5	L	;JUE
5	54.643/255/0	L	;VIE
6	236.79/255/0	L	;SAB
7	255/91.071/0	B	;DGO
N	128

For days we also check the setting of TIME_WEEK_START.

[Esteban82]

It works.

BTW, the abbreviated form of Sunday in Spanish (DGO) is based on an ISO norm or something like that? Because for me the correct form is "DOM".

[PaulWessel]

Not sure where the DGO came from. Some googling cannot confirm that but I find DOM. So perhaps open a bug report on this, @Esteban82 ?

[Esteban82]

So perhaps open a bug report on this, @Esteban82 ?

Ok, or maybe I can create a PR to modifie this file, right?
https://github.com/GenericMappingTools/gmt/blob/32296e512b173cfdd2c3e8da1d2735e49b79f02a/share/localization/gmt_es.locale

[PaulWessel]

Yep, that is fine.