Add latitude_true_scale to ccrs.LambertCylindrical

[shoyer]

This allows for plotting other popular cylindrical projections (e.g., Gall-Peters)

Fixes #1098

Example usage:

import matplotlib.pyplot as plt
import cartopy.crs as ccrs

plt.figure(figsize=(9.42477796077, 6))
ax = plt.axes(projection=ccrs.LambertCylindrical(latitude_true_scale=45))
ax.coastlines(resolution='110m')
ax.gridlines()

[CLAassistant]

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[shoyer]

I think the tests will need to be updated to add new cached images. Is that something I can do, or that happens automatically?