Fix invalid CUDA device function error in HPMG

[AlexanderSinn]

Replace template and if constexpr with good old overloading for NVCC 11.4

From Severin:

We found that the laser envelope crashes on Maxwell for the following input script:

my_constants.kp_inv = 10.e-6
my_constants.kp = 1. / kp_inv
my_constants.wp = clight * kp
my_constants.ne = wp^2 * m_e * epsilon0 / q_e^2

max_step = 50
hipace.dt = 10.*kp_inv/clight

hipace.verbose = 0

amr.n_cell = 512 512 1000

hipace.do_tiling = 0

geometry.prob_lo     = -25.6*kp_inv   -25.6*kp_inv   -13.*kp_inv
geometry.prob_hi     =  25.6*kp_inv    25.6*kp_inv    0.*kp_inv

laser.a0 = 2.5
laser.use_laser = 1
laser.position_mean = 0. 0. -4*kp_inv
laser.w0 = 3.2*kp_inv 3.2*kp_inv
laser.L0 = 2.26*kp_inv * .57 # .57 is a rough conversion factor between cos**2 and exp profiles
laser.lambda0 = 2*pi/34*kp_inv
laser.solver_type = multigrid

hipace.bxby_solver=explicit

amr.blocking_factor = 2
amr.max_level = 0

hipace.output_period = 1

hipace.numprocs_x = 1
hipace.numprocs_y = 1

# hipace.depos_order_xy = 2

geometry.coord_sys   = 0                  # 0: Cartesian
geometry.is_periodic = true  true  false  # Is periodic?

beams.names = no_beam

plasmas.names = plasma
my_constants.zramp = 20*kp_inv
plasma.density(x,y,z) = ne * if(z>zramp,1,z/zramp) * if(z>0,1,0)
plasma.ppc = 2 2
plasma.u_mean = 0.0 0.0 0.
plasma.element = electron
plasma.radius = 23.*kp_inv

diagnostic.diag_type = xz

with the following error:

amrex::Abort::0::GPU last error detected in file /home/diederse/software/hipace/build/_deps/fetchedamrex-src/Src/Base/AMReX_GpuLaunchFunctsG.H line 868: invalid device function !!!
SIGABRT
See Backtrace.0 file for details

This PR fixes the issue

• 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

[SeverinDiederichs]

I confirm that this indeed fixes the issue.