* apply @simonpintarelli proposed code change

* get rid of one more tempale

* default: -> throw, whitespace

* fix

* apply format

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Co-authored-by: Simon Pintarelli <simon.pintarelli@cscs.ch>

std::filesystem is part of C++17 which is required via CXX_STANDARD, there is no
need to check for it explicitly

* CI: rebuild base images / pin spack to v0.21
* CI: use anonymous environments instead of dev-build with SPEC
* avoid creating partitions with empty intervals in COSTA `custom_layout`



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Co-authored-by: Anton Kozhevnikov <toxa81@gmail.com>

* minor cleaups
* apply format

Introduce a few fixes for the scalar-relativistic radial solver.

Use a different sequence of operations to compute G+q augmentation charge.