#pragma GCC breaks CLANG and older GCCs

[StefanBruens]

This pragma is incompatible with older GCCs and CLANG:

necpp/src/nec2cpp.cpp

Lines 590 to 593 in 870f110

	#pragma GCC diagnostic push
	#pragma GCC diagnostic ignored "-Wstringop-truncation"
	strncpy( gm, "EN", 2 );
	#pragma GCC diagnostic pop

[   56s] nec2cpp.cpp: In function 'int readmn(FILE*, FILE*, char*, int*, int*, int*, int*, nec_float*, nec_float*, nec_float*, nec_float*, nec_float*, nec_float*)':
[   56s] nec2cpp.cpp:591:32: error: unknown option after '#pragma GCC diagnostic' kind [-Werror=pragmas]
[   56s]  #pragma GCC diagnostic ignored "-Wstringop-truncation"

The correct approach is to use strncpy(gm, "EN", 3); or memcpy(gm, "EN", 2); and remove the #pragma.

And the function signature can be changed to
int readmn(FILE*, FILE*, char[3], int*, int*, int*, int*, nec_float*, nec_float*, nec_float*, nec_float*, nec_float*, nec_float*).

This allows the compiler to check the array bounds at compile time.