Proposal: passing arguments to backends

[pablooliveira]

Verificarlo is preparing to move fully towards the interflop interface (verificarlo/verificarlo#93).

In this context we need a way to pass arguments and options to our backends. For example, the mca or random-rounding backends require options to configure the precision or rounding mode.

This is a proposal to pass arguments to the backends by adding two parameters to the interflop_init function with the following conventions,

/* interflop_init: called at initialization before using a backend.
 * It returns an interflop_backend_interface_t structure with callbacks
 * for each of the numerical instrument hooks.
 *
 * argc: number of arguments passed to the backend
 *
 * argv: arguments passed to the backend, argv[0] always contains the name of
 * the backend library. argv[] may be deallocated after the call to
 * interflop_init. To make it persistent, a backend must copy it.
 *
 * context: the backend is free to make this point to a backend-specific
 * context. The frontend will pass the context back as the last argument of the
 * above instrumentation hooks.
 * */

struct interflop_backend_interface_t interflop_init(int argc, char **argv,
                                                    void **context);

In this way, earch argument is free to parse argc, argv using well established libraries (such as argp.h). See example here in the ongoing verificarlo's interflop PR.

In this PR, I have only modified interflop.h to start the discussion. If this proposal is accepted, we should update the existing backends and the C++ interface before merging.

[pablooliveira]

@Helios77760 please feel free to comment on this !