OIFS time-stepping

[timofeymukha]

For now, collecting relevant materials here.

• https://www.mcs.anl.gov/~fischer/nek5000/oifs.pdf
• https://publications.anl.gov/anlpubs/2017/12/140626.pdf
• https://dl.acm.org/doi/abs/10.1007/BF01063118
• Nek5000 implementaion: advchar -> char_conv -> char_conv1

[Kamyarmvn]

Two other relevant material:

• https://www.mech.kth.se/~pschlatt/Nek5000_2021.pdf

• https://link.springer.com/article/10.1007/BF01063118