Allow to read numerical initial data for ScalarTensor

[guilara]

Proposed changes

Requires: #6154

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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 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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Further comments

[guilara]

@nilsvu This is what I briefly mentioned to you