[mini] print precision in output

[SeverinDiederichs]

This PR prints the precision in the output, so one knows, if the simulation was run in single or double precision:

Initializing CUDA...
CUDA initialized with 1 GPU per MPI rank; 1 GPU(s) used in total
MPI initialized with 1 MPI processes
MPI initialized with thread support level 0
AMReX (21.07-55-gb87ec0a8fc08) initialized
HiPACE++ (f05da101a027) running in single precision

Initializing CUDA...
CUDA initialized with 1 GPU per MPI rank; 1 GPU(s) used in total
MPI initialized with 1 MPI processes
MPI initialized with thread support level 0
AMReX (21.07-51-g5619ef246175) initialized
HiPACE++ (ec18bf7a9c74) running in double precision

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable