Proposed changes

Template parts of boundary condition class on elliptic::analytic_data::InitialGuess instead of elliptic::analytic_data::AnalyticSolution, so that classes that aren't exactly analytic solutions to a given problem can still use them.

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Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
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• The PR lists upgrade instructions and is labeled bugfix or
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