Expose version numbers in C API

[WardBrian]

Mentioned by @aseyboldt in #95. This adds three declarations to bridgestan.h:

extern int bs_major_version;
extern int bs_minor_version;
extern int bs_patch_version;

Which are populated by the version number macros we already have in version.hpp. Previously, these versions were only available by pulling apart the string returned by bs_model_info().

This also adds a warning to the Python interface if the model version and python library version differ. This should be rare, but good to check. We can do something similar in Julia in 1.9+ when it is released: pkgversion

[roualdes]

Looks good to me. Thanks.