IA: generate sourceResource.title correctly (fixes #7708)

Given IA metadata like the following:
```json
{"title": ["Foo", "bar"], "volume": "1880"}
```
IAMapper should generate sourceResource.title as such:
```json
{"sourceResource": {"title": ["Foo, 1880", "bar, 1880"]}}
```

See https://issues.dp.la/issues/7708
These two lines could be more clear with named arguments, like:
```
volumes = self._generate_map_dict("volume", from_props=None, via_map_meta=False)
```

Could the parameter say something about the desired behavior?  It requires some code-reading to figure out what _map_meta is about in order to know how to call the function.  (I.e. what does _map_meta do?  That could be the name of the argument.)