GET LINEWIDTH reports unknown keyword

[esternin]

... and SET LINEWIDTH seems to set some but not all line thicknesses (e.g. not the XAXISLINEWIDTH and YAXISLINEWIDTH). Help implies this was intentional:

SET LINEWIDTH is a shorthand way to set CURVELINEWIDTH and PLOTSYMBOLLINEWIDTH. 

We can change it to set all other linewidths as well, or leave it as is, since the hardcopy generation should no longer require scaling of linewidths when done by wx calls. However, we must change the error in GET LINEWIDTH to something like this:

LINEWIDTH` is an umbrella keyword, ambiguous. Please use CURVELINEWIDTH or PLOTSYMBOLLINEWIDTH

[esternin]

Keep code as is, clarify in Help file: see https://github.com/esternin/eXtrema/commit/3862cdbd441e6b1a161ce96f4cbc55c4fa91446b