Add an implicit conversion to double for unitval type.

Allow us to assign a unitval to a double, stripping off the units in
the process.  This isn't technically necessary for adding the
additional RF reporting; however, it's a small enough change that I
thought I'd piggyback it here.

Also, remove an unnecessary line in unitval::unitval(double,
unit_types).  Also a small change.
Add GETDATA capability for all forcing contributions.

The forcings added were CO2, CH4, N2O, H2O, O3, BC, OC, SO2 (direct,
indirect, and combined), and volcanic.  Total forcing and the
halocarbons were already in.
Merge pull request #31 from JGCRI/rf-reporting

Rf reporting