Complete CHANGELOG for 3.4

The following merged MRs are worth to be considered for mentioning in the changelog for 3.4. Please have a look and add their content in a few words if necessary, and then mark it in the checkbox.

• !2408 (merged)
• !2569 (merged)
• !2296 (merged)
• !2507 (merged)
• !2285 (merged) (We want to include this new grid variable concept after this can be really used instead of still in namespace 'Experimental')
• !2521 (merged)
• !2510 (merged)
• !2509 (merged)
• !2505 (merged)
• !2243 (merged)
• !2239 (merged) The LinearPDESolver can reuse the matrix and thus avoid unnecessary reassembly. See test/porousmediumflow/tracer/constvel/main.cc for an example.
• !2414 (merged)
• !2454 (merged)
• !2421 (merged)
• !2457 (merged)
• !2422 (merged)
• !2419 (merged) : This MR is mentioned in the changelog for 3.3. Please check if this is correct.
• !2425 (merged)
• !2282 (merged) (This was not merged into masterbut into the new staggered dev branch)
• !2417 (merged)