Wrap related items in 'record' tags

The register of members' financial interests uses lots of these:
```
<p class="spacer"><p>
```
…to separate different interest records. parlparse [currently ignore this information](https://github.com/mysociety/parlparse/blob/86451ea9/pyscraper/regmemfilter.py#L69), but it might be quite useful.

Note that this _does_ modify the structure of the output regmem xml.