Clear Namespace DB on subsequent exports.

[hughjackson]

In some projects it may be useful to generate a full SoC UVM output and also multiple Block Level outputs. I have tried this and, with the default options, the classes don't get declared in later packages. I believe this is because of namespace_db which prevents class redeclaration. I think it would be good to add an option, to allow clearing of namespace_db at the start of the export method. Effectively each export would be considered unique. This can be worked around by creating a new exporter between exports, but this issue did confuse me for a little bit. Not sure if it is worth fixing, but worth clarifying somewhere