Add missing include

[mocknen]

Description

Summary:

assert is used in kdtree_cpu.cpp but <cassert> is not included directly.
It seems to be included through Eigen/Core, but this does not appear to be the case in the latest file, which causes the build to fail.

Changes and type of changes (quick overview):

• Include <cassert>

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Checklist:

Code related

• I have made corresponding changes to the documentation
  (i.e.: function, class, script header, README.md)
• I have commented hard-to-understand code
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
  (Check contributing/contributing_instructions.md for local testing procedure using libnabo-build-system)

PR creation related

• My pull request base ref branch is set to the develop branch
  (the build-system won't be triggered otherwise)
• My pull request branch is up-to-date with the develop branch
  (the build-system will reject it otherwise)

PR description related

• I have included a quick summary of the changes
• I have indicated the related issue's id with # <issue-id> if changes are of type fix
• I have included a high-level list of changes and their corresponding types
  (See contributing/commit_msg_reference.md for details)

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