Add better description of node placement in walls and targets for conduction calculation

[rpeacoc]

From a user:

"the CFAST TRG explains that the distribution of nodes in a target depends on the penetration depth which depends on the time t-end.

Could you explain how t-end is calculated (I did not observe any effect by changing the simulation time or the fire duration)?

How many nodes are used inside a cylindrical/plate target and how are they distributed depending of X-p?"