Create get_empty_interferometer function

Puts the creation of inteferometers into a function. In theory, this
means we don't need to instantiate them all at `import`, but for
backward compatibility I have included them.

peyote/detector.py

@@ -304,20 +304,40 @@ class PowerSpectralDensity:
         return frequency_domain_strain, frequency
 
 
-# Detector positions taken from LIGO-T980044-10 for L1/H1 and from arXiv:gr-qc/0008066 [45] for V1/ GEO600
-H1 = Interferometer(name='H1', power_spectral_density=PowerSpectralDensity(), length=4,
-                    latitude=46 + 27. / 60 + 18.528 / 3600,
-                    longitude=-(119 + 24. / 60 + 27.5657 / 3600), elevation=142.554, xarm_azimuth=125.9994,
-                    yarm_azimuth=215.994, xarm_tilt=-6.195e-4, yarm_tilt=1.25e-5)
-L1 = Interferometer(name='L1', power_spectral_density=PowerSpectralDensity(), length=4,
-                    latitude=30 + 33. / 60 + 46.4196 / 3600,
-                    longitude=-(90 + 46. / 60 + 27.2654 / 3600), elevation=-6.574, xarm_azimuth=197.7165,
-                    yarm_azimuth=287.7165,
-                    xarm_tilt=-3.121e-4, yarm_tilt=-6.107e-4)
-V1 = Interferometer(name='V1', power_spectral_density=PowerSpectralDensity(psd_file='AdV_psd.txt'), length=3,
-                    latitude=43 + 37. / 60 + 53.0921 / 3600, longitude=10 + 30. / 60 + 16.1878 / 3600,
-                    elevation=51.884, xarm_azimuth=70.5674, yarm_azimuth=160.5674)
-GEO600 = Interferometer(name='GEO600', power_spectral_density=PowerSpectralDensity(asd_file='GEO600_S6e_asd.txt'),
-                        length=0.6, latitude=52 + 14. / 60 + 42.528 / 3600, longitude=9 + 48. / 60 + 25.894 / 3600,
-                        elevation=114.425,
-                        xarm_azimuth=115.9431, yarm_azimuth=21.6117)
+def get_empty_interferometer(name):
+    """ Detector positions taken from LIGO-T980044-10 for L1/H1 and from
+        arXiv:gr-qc/0008066 [45] for V1/ GEO600
+    """
+    if name == 'H1':
+        H1 = Interferometer(name='H1', power_spectral_density=PowerSpectralDensity(), length=4,
+                            latitude=46 + 27. / 60 + 18.528 / 3600,
+                            longitude=-(119 + 24. / 60 + 27.5657 / 3600), elevation=142.554, xarm_azimuth=125.9994,
+                            yarm_azimuth=215.994, xarm_tilt=-6.195e-4, yarm_tilt=1.25e-5)
+        return H1
+    elif name == 'L1':
+        L1 = Interferometer(name='L1', power_spectral_density=PowerSpectralDensity(), length=4,
+                            latitude=30 + 33. / 60 + 46.4196 / 3600,
+                            longitude=-(90 + 46. / 60 + 27.2654 / 3600), elevation=-6.574, xarm_azimuth=197.7165,
+                            yarm_azimuth=287.7165,
+                            xarm_tilt=-3.121e-4, yarm_tilt=-6.107e-4)
+        return L1
+    elif name == 'V1':
+        V1 = Interferometer(name='V1', power_spectral_density=PowerSpectralDensity(psd_file='AdV_psd.txt'), length=3,
+                            latitude=43 + 37. / 60 + 53.0921 / 3600, longitude=10 + 30. / 60 + 16.1878 / 3600,
+                            elevation=51.884, xarm_azimuth=70.5674, yarm_azimuth=160.5674)
+        return V1
+    elif name == 'GEO':
+        GEO600 = Interferometer(name='GEO600', power_spectral_density=PowerSpectralDensity(asd_file='GEO600_S6e_asd.txt'),
+                                length=0.6, latitude=52 + 14. / 60 + 42.528 / 3600, longitude=9 + 48. / 60 + 25.894 / 3600,
+                                elevation=114.425,
+                                xarm_azimuth=115.9431, yarm_azimuth=21.6117)
+        return GEO600
+    else:
+        raise ValueError('Interferometer {} not implemented'.format(name))
+
+
+# Maintain backward compatibility - should be removed in the future
+H1 = get_empty_interferometer('H1')
+L1 = get_empty_interferometer('L1')
+V1 = get_empty_interferometer('V1')
+GEO600 = get_empty_interferometer('GEO600')