Implement general getter for C-API

[awvwgk]

Having a general getter in the C-API would allow to retrieve values from opaque types. Since the actual type is unknown, the storage location the value should be written to must be passed as void*, as safety measure a positive integer of the available number of bytes can be passed (in case NULL is passed we assume void* has enough memory to hold any result). An error is created if the key does not correspond to any entry or if the required memory is larger than the available.

A possible API could be given provided with

DFTD4_EXTERN void DFTD4_API_CALL
dftd4_structure_get(dftd4_error, dftd4_structure, char key[], void* value, int64_t size);

DFTD4_EXTERN void DFTD4_API_CALL
dftd4_param_get(dftd4_error, dftd4_param, char key[], void* value, int64_t size);

DFTD4_EXTERN void DFTD4_API_CALL
dftd4_model_get(dftd4_error, dftd4_model, char key[], void* value, int64_t size);

#define dftd4_get(error, ptr, ...) \
  _Generic((ptr),                  \
    dftd4_structure                \
    : dftd4_structure_get,         \
    dftd4_model                    \
    : dftd4_model_get,             \
    dftd4_param                    \
    : dftd4_param_get              \
    )((error), (ptr), __VAR_ARGS__)

Could be a solution to peek into the damping parameters if created by method name, cc Lori @loriab. Not sure whether a generic setter is in-scope for dftd4 at the moment. Thanks to Miguel @steinmig for the suggestion.