v0.5.21

Trixi v0.5.21

Diff since v0.5.20

Closed issues:

• Replace hardcoded Float64 in MPI buffers with uEltype (#910)

Merged pull requests:

• Replace hardcoded Float64 in MPI buffers with uEltype (#1429) (@ArseniyKholod)
• Fix run folder instructions in README.md (#1430) (@amrueda)