Add command-line parameter to skip the deletion of reference entities

When using running Initializer for a country for which knowingly no reference entities and reference assets exist in the database, calling the two functions

if initializer.exposure_db.exposure_exists(country_iso_code, reference_exists=True):
    initializer.exposure_db.remove_exposure(country_iso_code, delete_reference=True)

can be skipped to save time. Therefore, I suggest to add a new command-line parameter --skip-deletion. It seems good that this parameter only exist in the long version so that unintentional misuse can be excluded.

\rfc @laurens @gislars @kasra