Point directly to where gridcorner can be installed from

Closes #9

examples/grid_examples/grid_F0F1F2.py

 import pyfstat
 import numpy as np
 import matplotlib.pyplot as plt
-from projection_matrix import projection_matrix
+
+try:
+    from gridcorner import gridcorner
+except ImportError:
+    raise ImportError(
+        "Python module 'gridcorner' not found, please install from "
+        "https://gitlab.aei.uni-hannover.de/GregAshton/gridcorner")
 
 F0 = 30.0
 F1 = 1e-10
@@ -53,6 +59,6 @@ twoF = search.data[:, -1].reshape((len(F0_vals), len(F1_vals), len(F2_vals)))
 xyz = [F0_vals, F1_vals, F2_vals]
 labels = ['$f - f_0$', '$\dot{f} - \dot{f}_0$', '$\ddot{f} - \ddot{f}_0$',
           '$\widetilde{2\mathcal{F}}$']
-fig, axes = projection_matrix(twoF, xyz, projection='log_mean', labels=labels,
-                              whspace=0.1, factor=1.8)
+fig, axes = gridcorner.gridcorner(
+    twoF, xyz, projection='log_mean', labels=labels, whspace=0.1, factor=1.8)
 fig.savefig('{}/{}_projection_matrix.png'.format(outdir, label))

pyfstat/grid_based_searches.py

@@ -782,10 +782,15 @@ class EarthTest(GridSearch):
                   r'$\Delta P_\mathrm{spin}$ [min]',
                   r'$2\mathcal{F}$']
 
-        from projection_matrix import projection_matrix
-
-        fig, axes = projection_matrix(data, xyz, projection=projection,
-                                      factor=1.6, labels=labels)
+        try:
+            from gridcorner import gridcorner
+        except ImportError:
+            raise ImportError(
+                "Python module 'gridcorner' not found, please install from "
+                "https://gitlab.aei.uni-hannover.de/GregAshton/gridcorner")
+
+        fig, axes = gridcorner(data, xyz, projection=projection, factor=1.6,
+                               labels=labels)
         axes[-1][-1].axvline((lal.DAYJUL_SI - lal.DAYSID_SI)/60.0, color='C3')
         plt.suptitle(
             'T={:.1f} days, $f$={:.2f} Hz, $\log\mathcal{{B}}_{{S/A}}$={:.1f},'