Fix bofek

[TomHottentot]

I've fixed and tested BOFEK function as discussed in #541

py7zr is also no longer needed.

I also have a nlmod/data/bofek/bofek.json which looks like this but is blocked by git ignore. It's based on the provided SLD file but perhaps it's cleaner to split the label by the 4 letter code and text before and after colon. Let me know if you want that as well and how best to share.

[
    {
        "value": 1001,
        "label": "1001 Dikke veengronden: kleiig moerige bovengrond op bos-/zeggeveen",
        "color": "#a900e6"
    },
    {
        "value": 1002,
        "label": "1002 Dunne veengronden: moerige bovengrond op veen op zand zonder podzol",
        "color": "#5970ff"
    },
]