Add parameter to select S1 platform identifier

To select the identifier of the platform (S1A or S1B), we need to add a parameter in the config file.

[DataSource]
platform=S1A

This parameter is optional. Value can be "S1A", "S1B" "S1C" or "S1D", or a list of these values, separated by a comma.

In the S1FileManager.py, we just need to add a filter like that:

        # Filter relative_orbits -- if it could not be done earlier in the search() request.
        if len(relative_orbit_list) > 1:
            filtered_products = []
            for rel_orbit in relative_orbit_list:
                filtered_products.extend(products.filter_property(relativeOrbitNumber=rel_orbit))
            products = filtered_products

        # Filter platform .
        if len(platform_list) > 1:
            filtered_products = []
            for platform in platform_list:
                filtered_products.extend(products.filter_property(platformSerialIdentifier=platform ))
            products = filtered_products