Rename insurance_path config key to tiff_path (insurance_path still accepted)

Renamed in engine.ps1, both EXAMPLE configs, and the README. Existing
entity.json/practice.json files using insurance_path keep working - the
engine copies it to tiff_path after the merge when tiff_path isn't set.

Entities/EXAMPLE/Export1/PRACTICE_NAME/practice.json

@@ -4,7 +4,7 @@
   "ftps": {
     "username": "practice-specific-username",
     "password": "practice-specific-password",
-    "insurance_path": "/{practice}/CustomInsuranceFolder",
+    "tiff_path": "/{practice}/CustomInsuranceFolder",
     "pdf_path": "/{practice}/CustomPdfFolder"
   },
 

Entities/EXAMPLE/entity.json

@@ -8,7 +8,7 @@
     "tls": true,
     "username": "shared-username",
     "password": "shared-password",
-    "insurance_path": "/{practice}/Insurance",
+    "tiff_path": "/{practice}/Insurance",
     "pdf_path": "/{practice}/PDF"
   },