Technical rework of HND Bayern harvester

Some remarks in no particular order:

• Functions fetch_links and fetch_hnd just called once with their result forwarded, i.e. they do not structure anything and can be removed.
• Filter all_links before deduplication to avoid sorting/deduplicating irrelevant entries.
• Use an enum type Parameter to avoid unchecked matching on stringly-typed data, i.e. make invalid states irrepresentable.
• to_string is almost always "wrong": It is used to convert types into a human-readable representation, e.g. numbers into text. To turn a string slice into an owned text, meaning text into another kind of text, to_owned should be preferred.
• Sites is a structure describe a single site, hence it is renamed to Site. But the variable containing a list of sites is still sites of type Vec<Site>.
• In regular expression, use capture groups to capture exactly what is required, nothing more. This avoids any post-processing using e.g. replace.
• If the regular expression contains a capture group which is always present if the regular expressed matched at all, there is no need for error handling, i.e. captures[index] or captures.get(index).unwrap() can be used.
• Reduce the scope of bindings as much as possible and avoid making them mutable, e.g. prefer let region = if let Some(muni) = muni { Region::Other(muni) } else { Region::BY} to let mut region = Region::BY; if let Some(muni) = muni { region = Region::Other(muni); } as the first form is clearer as it indicates that there can be no more changes later on.
• Avoid producing owned types like String if they will just be format!ted into a larger text elsewhere to avoid needless allocations via calls to to_owned.
• When turning Option (e.g. the captures of a regular expression which might have matched or not) into Option (e.g. a site ID which might be missing), this can often be expressed more succinctly using the Option::map combinator. It is not just about less text though, as map immediately implies a standard mode of operation whereas pattern matching can express anything and the reader has to figure it out each time it is read.