String field type for ID and an extra option to prune unmatched trajectory

[cyang-kth]

• It could happen that the ID field of a trajectory is an extremely long id, which could be unsuitable for int parsing, considering change the type to String
• The current version will export unmatched trajectory as an empty cpath and linestring. Considering adding a bool flag to the configuration object to control this type of output (prune unmatched or not).

[cyang-kth]

• Another issue, if one trajectory is not matched, it is currently exported with traj id of 0. The real ID should be added to the current line.

https://github.com/cyang-kth/fmm/blob/5cccc608903877b62969e41a58b60197a37a5c01/src/mm/fmm/fmm_algorithm.cpp#L91