Reduce dependency tree memory consumption

[yahavi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.

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The following variables exist for every dependency tree node and can be removed to reduce RAM consumption:

• name
• groupId
• artifactId
• version

The following variables exist for every issue and can be removed to reduce RAM consumption:

• created
• issueType
• provider

Finally, all empty fields in GeneralInfo, Issue, and License will be ignored when saving to the file system.