Deviations from the upstream pybind11

[SomeoneSerge]

TLDR:

• pybind_utils.h assume different template signatures than upstream
• cmake links to pybind11 instead of pybind11::module, confusing cmake sometimes to try and -lpybind11 (which it needn't and which fails) instead of using the pybind11 cmake target
• cmake links to pybind11 even pybind11_add_module targets (these link automatically)

The pinned pybind11 submodule is relatively old. In the upstream pybind11 there already are provided some of the utilities from pybind_utils.h, like make_value_iterator, iterator_state, but they have different signatures (e.g. 5 template parameters against 4). I hoped to build OpenSfM without submodules with the (newer) pybind11 version pinned outside the repo, but this would require modifications to the template definitions

• Would you consider updating to the newer pybind11 before the next release?
• Do you need the in-tree pybind11 in the first place?