Track files: Support multi-line key-values

[Lestropie]

Issue arose due to potential for multi-line "command_history" field following #1603.

Closes #1925.

[Lestropie]

I'll need to also modify tckinfo to display such information properly.

[Lestropie]

$ tckedit 0.tck test1.tck -force
$ tckedit test1.tck test2.tck -force
$ tckedit test2.tck test3.tck -force
$ tckinfo test3.tck
***********************************
  Tracks file: "test3.tck"
    act:                  anat/5tt.mif
    backtrack:            1
    command_history:      tckedit 0.tck test1.tck -force  (version=3.0_RC4-421-g67ce65b4-dirty)
                          tckedit test1.tck test2.tck -force  (version=3.0_RC4-421-g67ce65b4-dirty)
                          tckedit test2.tck test3.tck -force  (version=3.0_RC4-421-g67ce65b4-dirty)
    count:                0
    crop_at_gmwmi:        1
    downsample_factor:    3
    fod_power:            0.25
    init_threshold:       0.1
    lmax:                 8
    max_angle:            45
    max_num_attempts:     1000000000
    max_num_tracks:       10000000
    max_seed_attempts:    50
    max_trials:           1000
    method:               iFOD2
    mrtrix_version:       3.0_RC4-421-g67ce65b4-dirty
    output_step_size:     1.25
    rk4:                  0
    samples_per_step:     4
    seed_dynamic:         diffusion/CSD8.mif
    sh_precomputed:       1
    source:               diffusion/CSD8.mif
    step_size:            1.25
    stop_on_all_include:  0
    threshold:            0.1
    timestamp:            1580346089.9433526993
    total_count:          0
    unidirectional:       0
    ROI:                  exclude ../diffusion/mask.mif

Does not yet have the ability to hide contents of properties with an egregiously large amount of lines as mrinfo does, but it's probably not high priority, it'll be far less likely to occur with such data.

[Lestropie]

P.S. @matteofrigo: This doesn't necessarily guarantee that these data will work with nibabel out of the box, since their code probably makes a similar assumption about key-values within track files being single-line. So if it still doesn't work after this fix it would be worth reporting to them. There is a chance that it'll "work", e.g. it will import only the final line of command_history; or it might fail because it detects a duplicated key as erroneous. Depends on the particulars of their code.

[matteofrigo]

@Lestropie It solved the problem and nibabel behaves as you predicted, i.e. it reads only the final line of command_history.

Thanks!
m

[Lestropie]

Excellent; thanks for the test!