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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Further comments

[guilara]

@nikwit I just rebased this. But haven't figured out yet how to include it in the CSW executable. I've only used it for the ScalarTensor system (which mimics the Gh system) and where we use the evolution::initial_data::InitialData class (e.g.

spectre/src/Evolution/Executables/GeneralizedHarmonic/EvolveGhBinaryBlackHole.hpp

Line 446 in e7e48dc

tmpl::pair<

).