Insonsistent data naming scheme across modules

Stemming from https://github.com/precice/openfoam-adapter/pull/197#discussion_r724973150. Check the adapter documentation.

Currently expected name prefixes:

• CHT
  • Temperature
  • Heat-Flux (snake-case)
  • Sink-Temperature
  • Heat-Transfer-Coefficient
• FSI
  • Force
  • Stress
  • Displacement
  • DisplacementDelta (CamelCase)
• FF (currently not documented :see_no_evil:)
  • Pressure
  • PressureGradient (CamelCase)
  • Velocity
  • VelocityGradient

This would be a breaking change (we would need to update the tutorials), so I would suggest that we choose one system and support both styles until v2.

In the contributing guidelines of preCICE we have already chosen the snake-case format. This would also be consistent with our precice-config.xml.

@DavidSCN what do you think?