[SIP] Change all occurences of convert[From|To]Instance to convert[From|To]Type

Removal of these deprecated calls is expected to improve stability

Author: m-kuhn

python/core/conversions.sip

@@ -1017,7 +1017,7 @@ template<TYPE1, TYPE2>
             if (!sipCanConvertToType(t1obj, sipType_TYPE1, SIP_NOT_NONE))
                 return 0;
 
-            if (!sipCanConvertToInstance(t2obj, sipClass_TYPE2, SIP_NOT_NONE))
+            if (!sipCanConvertToType(t2obj, sipType_TYPE2, SIP_NOT_NONE))
                 return 0;
         }
 

python/core/qgsfeatureiterator.sip

@@ -41,7 +41,7 @@ public:
 %MethodCode
   QgsFeature* f = new QgsFeature;
   if (sipCpp->nextFeature(*f))
-    sipRes = sipConvertFromInstance(f, sipClass_QgsFeature, Py_None);
+    sipRes = sipConvertFromType(f, sipType_QgsFeature, Py_None);
   else
   {
     delete f;

python/core/qgsfield.sip

@@ -168,7 +168,7 @@ class QgsFields
   {
     qDebug("__getitem__ %d", a0);
     QgsField* fld = new QgsField(sipCpp->at(a0));
-    sipRes = sipConvertFromInstance(fld, sipClass_QgsField, Py_None);
+    sipRes = sipConvertFromType(fld, sipType_QgsField, Py_None);
   }
 %End*/