Enhancement: add optional height scale for SkewT

[peters77]

Nice to have enhancement: an additional height scale derived from the pressure.
Example found in Metpy:

heights = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]) * units.km
std_pressures = mpcalc.height_to_pressure_std(heights)
for height_tick, p_tick in zip(heights, std_pressures):
    trans, _, _ = skew.ax.get_yaxis_text1_transform(0)
    skew.ax.text(0.02, p_tick, '---{:~d}'.format(height_tick), transform=trans)

Another idea could be adding hardcoded standard heights like this at the labeld isobares (taken from pymeteo):

def label_std_heights(axes):
"""Plot heights of standard pressure levels
:paramter axes: The axes to draw on
:type axes: :py:class:matplotlib.axes
"""
xpos = Tmin+1.5
std_heights = [(1000,111),(925,2512),(850,1457),(700,3012),(500,5574),(400,7185),
(300,9164),(250,10363),(200,11784),(150,13608)]#,(100,16180)]
for p,z in std_heights:
label(xpos, p, str(z), 'black', 0, axes)