Parametrizable namespace to GML output

[strk]

Rather than always add an 'gml:' namespace to GML output routine, allow overriding it, or completely omitting it.

[strk]

coliver, do you have plans to work on this ? or I'll proceed myself.

The requirement is to specify a custom prefix or no prefix at all. I guess we could take a null as a no-prefix (also exclude colon), a missing argument as the default ("gml") and anything else as the prefix to use (adding the colon char).

With this interface you would be able to specify an empty string prefix but still get the colon. Would that make sense at all ? (I'm not a GML user).

[strk]

The liblwgeom part is done as of r5452. Was a pain to go over that huge file

[strk]

Ready in r5459.