Try the methane history approach in the VOC code

In ch4Mod.F90 we use namelist flag `hist_wrtch4diag` to determine whether a long list of methane fields will appear in history:
```
if (hist_wrtch4diag) then
active_via_hist_wrtch4diag = "active"
else
active_via_hist_wrtch4diag = "inactive"
end if
```
We then assign each such field with `default=active_via_hist_wrtch4diag`, which gives users the flexibility to add a shorter list of these fields using a namelist fincl statement if they prefer not to see all the fields.

The VOC code generates a long list of fields, so consider the same approach.