Everything gets 0 / NaN if confidence is -1 in motChallenge format

[Sebastian-Vogt]

As stated in this issue #8 and in the comments following this one:
All the results get 0s or NaN, if one insterts GT data with -1 as confidence.
The mot challenge instructions state, that 0s should be ignored and any other value should be used.
They also use -1 in their example.

[jvlmdr]

It indeed seems like the original matlab toolkit excludes only gt rows with confidence == 0. See here:
https://github.com/dendorferpatrick/MOTChallengeEvalKit/blob/45098695e9c57999793181b658bebce380df6686/matlab_devkit/evaluateTracking.m#L50