Add observed depth constraint in bundle adjustment

[servantftransperfect]

This pull request adds support for handling and storing per-observation depth information throughout the Structure-from-Motion (SfM) pipeline. The changes propagate the new depth field in the Observation class, update serialization/deserialization logic, and integrate depth constraints into bundle adjustment. This enables the use of measured depth data (e.g., from RGB-D sensors) to improve reconstruction accuracy.

Depth support integration:

• Added a depth field to the Observation class, with getter/setter methods and documentation, allowing each observation to optionally store a measured depth value.
• Updated JSON and Alembic import/export logic to read, write, and propagate the new depth field for each observation. [1] [2] [3] [4] [5] [6] [7] [8]

Bundle adjustment improvements:

• Introduced a new cost function DepthErrorFunctor to enforce depth constraints in bundle adjustment, and added logic to include a depth residual for observations with valid depth measurements. [1] [2] [3]

Pipeline propagation:

• Ensured that triangulation and other pipeline stages correctly set and propagate the depth value in Observation objects.

Version update:

• Bumped the revision number in sfmDataIO.hpp to reflect changes.