Cmake fixes

[derpda]

Thank you for merging the last PR!
I was away on the weekend, so I apologize for the late response.

As promised, here is the PR addressing issues #8 and #9, making modifications to the CMake code.
With this PR, the configure step will

• Download Eigen according to the EIGEN_VERSION variable, here set to 3.3
• Properly set the INTERFACE_INCLUDE_DIRECTORIES on the bytetrack target

This makes it significantly easier to use ByteTrack from other CMake projects (as is my use case).

Side note

In the mean time, I have been making more drastic changes to the main code, since it looks like I will be using this library quite intensively.

Namely I have

• Enforced a consistent style using clang-format and committed that configuration file
• Refactored the main ByteTrack::update method for (what I think is) better readability
• Removed any use of new and delete for the lapjv invocation and replaced it with std::vector
• Removed the basically unused Object class, integrating it into the STrack class
• Removed any use of const int& or other constant primitive references, as these have no benefit

as well as some other minor changes.
None of those changes are in this PR, but if you are interested I could make additional PRs.

[medalotte]

Thanks! I've been very busy and am sorry for the delay in responding.