In some cases it would be helpful to parse comments from BNGL files

`bionetgen.bngmodel` nicely parses models. In some cases, it may be helpful to parse the comments out of files.

For example, the comment of `Vec` is currently parsed to `None`
```
begin parameters
 Vec   1000*Vcell # Volume of extracellular space around each cell (1/cell density)
end parameters
```

```python
model = bionetgen.bngmodel(...)
model.parameters['Vec'].comment
```