Expose +gamma for Oblique Mercator

[epn09]

Rationale

• I think it was agreed in Expose PROJ parameter +gamma for Oblique Mercator #2286 that it is fine to expose the gamma parameter of Oblique Mercator projection. However, I have not seen it implemented.

Implications

• It will be possible to make a rotated map like the example below.

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

japan_extent = [128, 146, 24, 46]
fig = plt.figure(figsize=(8, 8), layout="constrained")

ax1 = fig.add_subplot(
    1,
    2,
    1,
    projection=ccrs.ObliqueMercator(
        central_latitude=35, central_longitude=137, azimuth=40
    ),
)
ax1.coastlines()
ax1.gridlines(draw_labels=True)
ax1.set_extent(japan_extent, crs=ccrs.PlateCarree())

ax2 = fig.add_subplot(
    1,
    2,
    2,
    projection=ccrs.ObliqueMercator(
        central_latitude=35, central_longitude=137, azimuth=40, gamma=0
    ),
)
ax2.coastlines()
ax2.gridlines(draw_labels=True)
ax2.set_extent(japan_extent, crs=ccrs.PlateCarree())

plt.show()

[CLAassistant]

All committers have signed the CLA.