WIP: New PCT format

[acoussat]

First draft for the new PCT format. Probably incomplete but just to make sure I am on the right tracks. A few remarks in no particular order:

• The names I have given for the various fields are just a proposal, feel free to suggest alternatives.
• Should we add some kind of prefix for the keys of the dictionary to clearly identify that they come from PCT? For instance, instead of having UpstreamPositionU, we would have PCTUpstreamPositionU. It's heavier but could perhaps avoid potential conflicts?
• I think it would be helpful to have a separate page that specifies PCT's geometry and introduces the variables we use here ($u$, $v$, $w$, $\dot u$, $\dot v$, $\dot w$, etc.).