Update naming convention for discipline specific variables

[joanibal]

I'm adding an issue raised by others at the last meeting, so that it can be discussed further here.

The issue, as paraphrased by @kejacobson is

"So far we've assumed the thermal mesh is the same structural analysis mesh on the structures side, but in general that may not be true. We've got _a for aero variables, _s for structures, so we decided we need a new one for the thermal analysis."

and the current idea that came out of the meeting is

"Since we aren't limited to one letter, we thought we should clarify the existing descriptions too. So change existing variables with extensions: _a -> _aero , _s -> _struct, and add _therm for variables related the thermal problem"

[joanibal]

I like the idea of changing the existing extensions for aerodynamic and structural variables to be more descriptive.

However, I don't think all variables related to the thermal problem can use a single extension is the same way. This is because the thermal problem is actually two thermal disciplines, convection and conduction.

The meldThermal component for example transfers temperatures from the conduction mesh to the convection mesh. so with only a single extension naming convention the variables would be

                 -------------
temp_therm ---->| MeldThermal |  ---> temp_therm 
                 -------------

Two distinct extensions are needed to separate the temperature variables in the conduction and convection analysis.

                        -------------
temp_convec_therm ---->| MeldThermal |  ---> temp_conduc_therm
                        -------------

or simply

                -------------
temp_conv ---->| MeldThermal |  ---> temp_cond
                -------------

[kejacobson]

Implemented in #50