plotSPC(): better estimation of depth axis intervals, ylim

[dylanbeaudette]

Disconnect lower depth of ylim from depth axis init. plotSPC() will more closely track values specified in max.depth and/or max(x). This is generally a "good thing", but figures in aqp 2.x will have tighter clearances on the bottom of figures. A large amount of extra vertical space (aqp 1.x) was common when the deepest profile was just a little larger than a multiple of 50cm.

See updated example for harden.melanization(), esp:

# max(jacobs2000) is 213cm
# explicitly extend figure to 250cm
plotSPC(jacobs2000,
        color = "matrix_color",
        label = "melan",
        plot.order = jacobs2000$melanorder,
        max.depth = 250
        )