Allow `HSBoundaryExternalAppConvection` to use monomial variables

[joshuahansel]

Motivation

The usual setup for coupling to 2D heat structures from a 3D domain is to perform a layered average of the transfer quantity and transfer that into a variable in the heat structure application. Currently HSBoundaryExternalAppConvection creates FIRST LAGRANGE variables for the transferred temperature and heat transfer coefficient, but CONSTANT MONOMIAL is best when transferring from layered averages, assuming the layers match with the discretization of the heat structure. Therefore, there should be an option to switch the variable types.

Design

The following steps are suggested:

• Add the parameters T_ext_is_monomial and htc_ext_is_monomial to HSBoundaryExternalAppConvection. Make them default to false for now.
• Emit a deprecation warning if the user does not supply these parameters; tell them that the parameters will be required in the future. Recommend them to use true and tell them why.

Impact

New optional parameters in HSBoundaryExternalAppConvection. Users of this class will start getting deprecation warnings and be told to update their input files.

[joshuahansel]

For context, I used a MultiAppGeneralFieldUserObjectTransfer and transferred into these FIRST LAGRANGE variables and ended up with values that were noticeably different than the source field, and it wasn't even due to ambiguity of the source points due to equal-distance, since the source field was pretty axially uniform.