Add "Generic" Feature Type

[mh0g]

Description

Added a 'generic' feature type for importing features that are not computed by alicevision.

For now the descriptor size is set to 128, but may be extended and padded with 0 to support wider descriptors in the future.

[natowi]

Why not call it "Custom Feature Type"?

[fabiencastan]

@mh0g Could you rebase, to avoid merge nodes in the PR?

[fabiencastan]

Or "Generic128" ?

[MrClock8163]

Can I ask why you chose to limit it to 128?

[fabiencastan]

How would you do to get something fully dynamic?

[MrClock8163]

I meant the question as in, why limit it to such a low number to begin with? I understand that there has to be a limit, but why 128?
Granted, 128 is probably more than enough for control points, but if someone would want to import en-masse matches, that would be impossible this way.

[fabiencastan]

128 is for the memory size of each descriptor, not for the number of features/matches.

[MrClock8163]

I see. I suppose I misunderstood then. Figured this would work the same way as the CCTAG and AprilTag marker describers, where each match is a 128 bytes block, and FF byte in the block identifies the marker ID, which limits it to 128 different markers.
Sorry for the confusion.