Reuse image pair io functionality

[p12tic]

This PR intends to prevent #1279 from happening again until functional tests for full pipeline are written.

The root cause of #1279 was that operator<< was used in a place where exact file format is expected. It's hard to search for usages of operator<<, so embedding important functionality there is error-prone, as unsuspecting developer may think that the end consumer of the output is human, not other tools. Another cause for the bug was that there were multiple sources of truth on how to output pair list, so there was always likelihood of the accidentally making incompatible changes.

This PR solves the above problems by making image pairs to use a single format defined in imageMatchingComponent.

savePairs has been enhanced to write pairs in the same format as loadPairs accept, namely multiple image pairs per line.

All file IO functionality has been moved to separate functions to aid testing.

Finally, savePairs replaces operator<< for image pairs output.

[fabiencastan]

Nice cleanup, just need to include the namespace with the name of the module to keep the rule that we apply everywhere and we can merge it.

[p12tic]

@fabiencastan Addressed your comment.