Enable passing new sourceURLs to metadata

[CeresBarros]

Many of us have felt the need to update an input's sourceURL and at the moment this is not possible. The is means that the user needs to override the default object with an external object (via simInit(..., objects = list(...)) -- or create specific modules to prepare these objects.

Here's a possible solution:
Pass a list of sourceURL's to simInit(..., objects) that will be used to override the metadata, alongside with any object "replacements". Here's an example:

simInit(..., objects = list("sourceURL" = list("obj1"= <URLstring>, "obj2"= <URLstring>),
                            "obj3" = <obj>)