Add estimated conversion factor for leaf areas to release?

[RemkoDuursma]

For conifers, a.lf needs to be converted to be comparable (either projected, or (half)total area). We did this in baadanalysis - might be useful to avoid confusion to add a conversion factor to the BAAD release (or even as a second a.lf variable).

Another option is to add a function to baad.data to do the conversion (with control by the user).