v.transects: Add categories of input lines to layer 2 of output

[mwort]

This allows reference of the output transects to the original lines, which is essential in many applications of transects. The way to get the categories of the input lines into the transect table, also added to the docs:

v.transects input=lines output=transects transect_spacing=100
v.db.addtable transects columns="line_cat"
v.db.addtable transects layer=2
v.to.db transects query_layer=2 query_column=cat option=query column=line_cat

I have also updated the type parameter description to say that it determines the output type rather than the input as it says now.

[neteler]

Waking up tests by closing/re-opening PR...

[echoix]

Suggested change

	linecat = True if cats else ''
	linecat = True if cats else ""

For black

[mwort]

I've rebased this on the latest grass8 branch and addressed the comment. Would be great to get this merged.
Thanks,
Michel