[IMP-dev] unit cleanup

[Daniel Russel <drussel@gmail.com>]

The C++ unit classes have been cleaned up and a few new types have  
been added. Scalar went away as there is now a common base class for  
all units (so adding operations with floats is easy).

Attachment: units
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The usual caveats about SConscript files not including new headers.  
(IMP.i shouldn't)