Allow to read in numerical initial data for CurvedScalarWave

[guilara]

Proposed changes

Add actions to read in numerical initial data for the CurvedScalarWave system. These data can be obtained, for instance, from an elliptic solve of the Poisson equation.

To be combined in a future PR with reading in numeric data for the metric variables in the ScalarTensor system.

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• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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• The PR lists upgrade instructions and is labeled bugfix or
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