Add ability to fix variables.

New command line option: --fix_variables <file_name>.

I needed this for testing the ECIU algorithm, and realized that it may
be useful to modelers in general.  The file format is the same as the
default Temoa output.  For example:

    1000.0   V_Activity[1990,inter,day,IMPDSL1,1990]
       2.5   V_FlowOut[1990,inter,day,DSL,RHO,1980,RH]

Example usage:

    $ coopr_python  temoa.py  --fix_variables  analysis_fixed.sol  data.dat
NFC: be consistent in my laziness.

This is preparation for a refactoring of how variable values are
acquired.  Currently, if a modeler fixes variables, they will not be
shown in the output.  Next commit should correct that oversight.
Fix bug with fixed variables not shown in output.

The addition of --fix_variables showed a problem with the results of
Temoa: namely that fixed variables were not shown.  This fix is more
explicit in nature than the previous general approach to printing
whatever was returned by the solver.  Now, this is Temoa specific, but
it has been all along.  I just hadn't put that together the first few
go-rounds.
If no solution, stop processing.

In d10c1a324a8d0e3075b616a6bec1ab579043a47e, I changed a return to
output.write() call.  Bad me, because the early quit was intentional.
Sigh.  We could really use a test suite ... when I have the time.
Simplify objective calculation logic.

We are now well beyond Coopr 2, and with loaded results, we can
calculate the objective function rather than find it as solved by the
solver.  The results should always be the same, and I'm trusting so.
Meanwhile, we get a benefit of simpler logic.
Unify variable calculation and formatting.

With the new more robust variable value collection and printing, unify
the calculation code with the collection code.  It makes no sense
looping over the variable indices more than once.