This commit exposes a `bundle_adjustment` function that has a similar
functionality to the command line utility `colmap bundle_adjuster`.  One
use case it enables is to allow one use pycolmap to run bundle
adjustment after sparse reconstruction to refine the principal point, as
jointly refining the principal point[^principal_point_refinement] during
sparse reconstruction would make sparse reconstruction on some scenes to
fail.

Note that not all available members of `ceres::Solver::Options` are
exposed in this change, but only the ones modified by
`BundleAdjustmentOptions` in colmap[^modified_solver_options].  Besides,
the registered `CeresSolverOptions` uses `py::module_local()` to avoid
conflicts with downstream libraries.

Example usage
=============

```python
from pathlib import Path

import pycolmap

sparse_reconstruction_directory = Path("sparse/0")  # or just `... = "sparse/0"`

ba_opts = {
  "refine_principal_point": True,
  "solver_options": {
    "max_num_iterations": 256,
  },
}

pycolmap.bundle_adjustment(
  input_path=sparse_reconstruction_directory,
  output_path=sparse_reconstruction_directory,
  options=ba_opts,
)
```

Known problem
=============

Though creating a `pycolmap.BundleAdjustmentOptions` object with a
nested `dict` works as expected, i.e., the following examples work well:

```python
import pycolmap

ba_opts = pycolmap.BundleAdjustmentOptions({
  "refine_principal_point": True,
  "solver_options": {
    "max_num_iterations": 256,
  },
})
print(ba_opts.todict())  # runs successfully with user-specified option values

ba_opts = pycolmap.BundleAdjustmentOptions({
  refine_principal_point=True,
  solver_options={"max_num_iterations": 256},
})
print(ba_opts.todict())  # runs successfully with user-specified option values
```

it gives an error while trying to create a
`pycolmap.BundleAdjustmentOptions` object with its `solver_options` set
to a `pycolmap.CeresSolverOptions` object, i.e. the following gives an
error:

```python
import pycolmap

solver_options = pycolmap.CeresSolverOptions(max_num_iterations=256)
print(solver_options.todict())  # runs successfully with user-specified option values

ba_opts = pycolmap.BundleAdjustmentOptions({
  "refine_principal_point": True,
  "solver_options": solver_options
})  # RuntimeError: instance allocation failed: new instance has no pybind11-registered base types
```

The "new instance" mentioned in the RuntimeError message probably points
to the `CeresSolverOptions` added in this commit, but I'm not sure about
a way to solve this at the moment, plus, as reported above, using
`dict`s to specify the option values works properly and works around
this error.

[^principal_point_refinement]: <https://colmap.github.io/faq.html#frequently-asked-questions>
[^modified_solver_options]: <https://github.com/colmap/colmap/blob/3.8/src/optim/bundle_adjustment.h#L81-L89>

Signed-off-by: Gaoyang Zhang <gy@blurgy.xyz>