[FEATURE]: Ensure seamless use of atan() and strcmp() on GPU

[willend]

Is your feature request related to a problem? Please describe.

1. Our OpenACC solution does not provide a atan() function.

2. Our OpenACC does not currently allow use of the standard strcmp() in WHEN clauses and EXTEND blocks, but our drop-in replacement str_comp() works.

Describe the solution you'd like

1. We should define a wrapper function atan(x)=atan2(x,1.0) in mccode-r.c (#ifdef OPENACC only)

2. We should make a #define strcmp(…) str_comp(…)(#ifdef OPENACC only) that works both within the main generated TRACE function (where WHEN’s are evaluated) and double check / ensure a similar define is in place in any component TRACE function.