Additional path effects

[nawendt]

This will add the capability to plot both forming and weakening variations of the basic front types that are already implemented. You will certainly see weakening fronts being plotted on WPC frontal analyses and forecasts, though I do not recall the last time I saw a forming front plotted. WPC does indicate that they can plot them here and indicate there can be varying frontal character in surface bulletins here. This also adds a dryline boundary. These also appear in WPC graphical products. However, the coded versions of those same products do not explicitly specify them as drylines. Bottom line is you should be able to get a plot to look like WPC even if their coded products don't necessarily match the specificity of what their graphics have.

There are also a couple of niche weather map symbols: the ridge axis and the squall. These are more about filling out the toolbox for someone who might want to draw a whole weather map using MetPy than anything.

I tried to keep within the same framework that the basic front types had as much as I could. See what you think and we can perhaps improve them in some places.

• Tests added
• Fully documented

[dopplershift]

I'm happy to see this go in. There are just a few minor clean-ups. Also need to deal with the conflict due to the merge of the scalloped lines.

I really want to get this into the release (hopefully tomorrow 5/4), so if you don't have a chance to make these changes I'll take care of it.

[dopplershift]

Thanks @nawendt !