Introduce HeatSources

[masterleinad]

This is a step towards https://github.com/adamantine-sim/adamantine/issues/190. This pull request replaces std::vector<std::shared_ptr<adamantine::HeatSource<dim>>> with HeatSources that contains all heat sources as Kokkos::Views and loops over them in its member functions. With this approach, there is no dynamic polymorphism and the compiler could inline all function calls for the heat sources. In particular, this object would be usable on GPUs (apart from function annotations).

[masterleinad]

Retest this please.

[Rombur]

Let me know when you want another review on this.

[masterleinad]

Let me know when you want another review on this.

This could take another review now. Some notes:

• I create local copies on the host for the HeatSources object where necessary assuming that we want it by default to be in the memory space specified in the respective template argument for the other operators. It might make sense to keep it in host memory instead and only create device copies or we keep a member variable for both memory spaces (if they don't coincide).
• I needed to move a bunch of const members in the heat sources to make these classes copyable. I was running into some issues with static variables if used on the device as well. Thus, I moved the respective constants into member functions.
• The recent changes reusing scan file paths made the HeatSources code a little more complicated. We are now storing those std::strings in HeatSources directly as well and recreating the scan file segments from them if necessary. Since the ScanPath doesn't own the ScanPathSegments, reading the file is a static member instead of a member function named "extract_scan_paths". I'm happy to change it back to some other name like "read_file" again if preferred.
• I believe I addressed all other comments.