Examples: add examples for universal and ext-drift kriging

examples/21_extdrift_kriging.py

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+import numpy as np
+from gstools import SRF, Gaussian, krige
+
+# condtions
+drift_model = Gaussian(dim=1, len_scale=4)
+drift = SRF(drift_model, seed=1010)
+cond_pos = [0.3, 1.9, 1.1, 3.3, 4.7]
+ext_drift = drift(cond_pos)
+cond_val = ext_drift * 2 + 1
+# resulting grid
+gridx = np.linspace(0.0, 15.0, 151)
+grid_drift = drift(gridx)
+# spatial random field class
+model = Gaussian(dim=1, var=2, len_scale=4)
+krig = krige.ExtDrift(model, cond_pos, cond_val, ext_drift)
+krig(gridx, ext_drift=grid_drift)
+ax = krig.plot()
+ax.scatter(cond_pos, cond_val, color="k", zorder=10, label="Conditions")
+ax.plot(gridx, grid_drift, label="drift")
+ax.legend()

examples/22_universal_kriging.py

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+import numpy as np
+from gstools import SRF, Gaussian, krige
+
+# condtions
+drift_model = Gaussian(dim=1, var=0.1, len_scale=2)
+drift = SRF(drift_model, seed=101)
+cond_pos = np.linspace(0.1, 8, 10)
+cond_val = drift(cond_pos) + cond_pos * 0.1 + 1
+# resulting grid
+gridx = np.linspace(0.0, 15.0, 151)
+drift_field = drift(gridx) + gridx * 0.1 + 1
+# spatial random field class
+model = Gaussian(dim=1, var=0.1, len_scale=2)
+krig = krige.Universal(model, cond_pos, cond_val, "linear")
+krig(gridx)
+ax = krig.plot()
+ax.scatter(cond_pos, cond_val, color="k", zorder=10, label="Conditions")
+ax.plot(gridx, gridx * 0.1 + 1, label="linear drift")
+ax.plot(gridx, drift_field, "--", label="original field")
+ax.legend()