Adding support for multiple cameras in different subdirectories

[trybala]

In the multi cam setup, the code expects to point DIM to a directory containing (somewhere in the tree) subdirectories named "cam", from where it will infer the camera for each image (e.g., EuRoC structure). Camera IDs in the directory names should match the camera models' definitions in the config.

Default single-cam behavior should not be affected.

Tested with matching by custom pair definition.
Not tested with upright images or tiling. Tests not tested.