Make Diagnostic box selection more robust

[AlexanderSinn]

This PR fixes an issue where

amr.n_cell = 1024 1024 512
geometry.prob_lo     = -400e-6   -400e-6   -50e-6
geometry.prob_hi     =  400e-6    400e-6    50e-6

plasma_diag.patch_lo = -400e-6   -400e-6    50e-6
plasma_diag.patch_hi =  400e-6    400e-6    50e-6

would crash with the unhelpful error message

0::Assertion `rtmp > std::numeric_limits<ParticleReal>::lowest() && iters < maxiters' failed, file "/home/asinn/hipace/build/_deps/fetchedamrex-src/Src/Base/AMReX_Geometry.cpp", line 590 !!!
SIGABRT

because the plasma_diag patch was technically outside the simulation domain.
With this PR the input works and if modified (50e-6 -> 51e-6) would give the error

0::Assertion `domain.ok()' failed, file "/home/asinn/hipace/src/diagnostics/Diagnostic.cpp", line 412, Msg: Box for diagnostic object 'plasma_diag' is empty. Make sure that it intersects with the simulation domain!
Simulation: ((0,0,0) (1023,1023,511) (0,0,0))
Diagnositc: ((-2,-2,516) (1024,1024,518) (0,0,0))
Intersection: ((0,0,516) (1023,1023,511) (0,0,0)) !!!
SIGABRT

• 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

[MaxThevenet]

Thanks for the fix!