Consider rewriting TPH_TDRY in terms of Reichler's algorithm

[Ian Culverwell]

The ROPP7.0 beta reviewer, Torsten Schmidt, prefers Reichler's algorithm (based on pressure) to, er, Schmidt's algorithm (based on height).

Reichler's algorithm is already implemented for TPH_TEMP, so shouldn't be too hard if we can calculate the pressure. Surely dry pressure is good enough here, and assuming linear fall in T between levels we can estimate pdry easily as long as press_sfc (a lev2c field) is available.

Reichler's algorithm is certainly simpler than Schmidt's, so this is a good idea.

[Ian Culverwell]

This was in fact done for ROPP7.0.

Closing ticket.