Some fields in netpyne.metadata are outdated or invalid

@salvadord arbeitet bereits daran.

Seit 11.8.2021.

A card to collect possibly invalid netpyne field declaration

**Mechs**

`float` type but actually a dictionary

```python
"mechs": {
"label": "Mechanisms",
"help": "Dictionary of density/distributed mechanisms, including the name of the mechanism (e.g. hh or pas) and a list of properties of the mechanism (e.g. {'g': 0.003, 'e': -70}).",
"suggestions": "",
"hintText": "",
"type": "float"
},
```