Let psvelo accept CPT and a richer way of coloring symbols

[PaulWessel]

Description of proposed changes

So far, only a constant color fill was allowed for the symbols offered by psvelo. This PR allows the user to supply a CPT file and then use one of many criteria to look up the corresponding color. These are the improvements:

1. Accept a CPT file via new option -C.
2. New option -Z sets which attribute of the data (magnitude, specific component, or a user column) should be used to assign the color [Default is magnitude, i.e. vector magnitude or rotation].
3. Modifier +e to -Z switches the coloring from the symbol fill (i.e., -G domain) to the error part (i.e., -E), allowing the error ellipse or rotation uncertainty to be colored based on the error estimates (uncertainty in magnitude, rotation, etc.).
4. Allow -L to also append an optional secondary pen for the error symbol outlines [Default is same pen as -W].
5. Both -L and -W inherit the +cl|f modifer used in plot to assign CPT color to fills, pens, or both.

I have added a new tests exploring most of the combinations of color and fill; here is what the plot looks like: