add json tests; code cleanup;

tests/fslstats/test_preopIndexMaskMeanTimeSeries_t_K_m.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-m": 0.5266819608311916
+                },
+                {
+                    "-m": 0.5156517274568186
+                },
+                {
+                    "-m": 0.49233699466459285
+                },
+                {
+                    "-m": 0.5116614613561304
+                }
+            ]
+        },
+        {
+            "indices": [
+                {
+                    "-m": 0.5513619920395724
+                },
+                {
+                    "-m": 0.5131755568141885
+                },
+                {
+                    "-m": 0.49409089087108476
+                },
+                {
+                    "-m": 0.5092079986783641
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_preopIndexMaskMean_K_m.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-m": 1.078043952870764
+                },
+                {
+                    "-m": 1.0288272842710071
+                },
+                {
+                    "-m": 0.9864278855356776
+                },
+                {
+                    "-m": 1.0208694600344945
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_preopMeanTimeSeries_t_m.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-m": 0.503236498022452
+                }
+            ]
+        },
+        {
+            "indices": [
+                {
+                    "-m": 0.504035022270371
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_robust_r.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-r": [
+                        0.018357284367084503,
+                        0.982818067073822
+                    ]
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_roi_w.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-w": [
+                        0,
+                        10,
+                        0,
+                        10,
+                        0,
+                        10,
+                        0,
+                        2
+                    ]
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_stdevNonzero_S.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-S": 0.2949828225878523
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_stdev_s.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-s": 0.29498282199940035
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_threshold_l_0.25_u_0.75_m.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-m": 0.49646345829352356
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_volumeNonzero_V.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-V": [
+                        2000.0,
+                        2000.0
+                    ]
+                }
+            ]
+        }
+    ]
+}

tests/fslstats/test_volume_v.json

+{
+    "volumes": [
+        {
+            "indices": [
+                {
+                    "-v": [
+                        2000.0,
+                        2000.0
+                    ]
+                }
+            ]
+        }
+    ]
+}