What is wrong?

• After London changes, certain genesis header params became unconfigurable. Introduce back the ability to configure block_number. See eth-tester issue #225.

How can it be fixed

• fill_header_params_from_parent() needs the ability to pass in a block_number kwarg to be configurable again. This call is made from the create_header_from_parent() method in the header classes.

• block_number should be a parameter in the fill_header_params_from_parent() method that can be configured but should still keep the current default to GENESIS_BLOCK_NUMBER if the parent is None or to the parent block_number + 1 if the parent exists.

At a quick glance I believe this is the only change necessary. Testing should be added as well.