Add gitignore entries for IntelliJ
Add gitignore entries for IntelliJ

Since my main Java IDE is IntelliJ, this seemed like a sane thing to
add.

ldbc/ldbc_graphalytics,ldbc/ldbc_graphalytics,tudelft-atlarge/graphalytics,ldbc/ldbc_graphalytics,tudelft-atlarge/graphalytics,tudelft-atlarge/graphalytics,tudelft-atlarge/graphalytics,ldbc/ldbc_graphalytics,ldbc/ldbc_graphalytics,tudelft-atlarge/graphalytics