Cannot have stop in accelerator region

[sbryngelson]

PR #749 introduces several impermissible statements, at least on AMD GPU hardware and with Cray compilers (v >= 19). The problem is stop statements in the GPU kernels. Examples are here:

MFC/src/simulation/m_bubbles_EL.fpp

Line 992 in 131b41e

stop "Check cluterflag. Exiting."

MFC/src/simulation/m_bubbles_EL.fpp

Line 1127 in 131b41e

if (intfc_rad(k, 1) <= 0._wp) stop "Negative bubble radius encountered, please reduce dt"

MFC/src/simulation/m_bubbles_EL.fpp

Line 1115 in 131b41e

if (intfc_rad(k, 2) <= 0._wp) stop "Negative bubble radius encountered, please reduce dt"

MFC/src/simulation/m_bubbles_EL.fpp

Line 1102 in 131b41e

if (intfc_rad(k, 2) <= 0._wp) stop "Negative bubble radius encountered, please reduce dt"

and so on and so forth.