Documentation: heterogeneous configuration documentation

[rafmudaf]

The heterogenous configuration functionality has gone through some iteration since its initial introduction to FLORIS v3. At this point, it is well settled, but no written documentation exists describing its use and modeling considerations. The Background and Concepts section of the online documentation serves as a general reference for teaching new users about fundamental concepts within the FLORIS software. It would be helpful to include a description of the idea of a heterogeneous atmospheric condition plus details of the implementation in FLORIS. Additionally, the flow_field.heterogenous_inflow_config block should be described in the input file reference.

TODO:

• Describe the background context of the heterogenous condition - what is it and why would anyone use it?
• Describe modeling considerations for this functionality - what would a user need to know in order to use this effectively?
• Document the flow_field.heterogenous_inflow_config input block in the input file reference

[rafmudaf]

Part of the description was included in #716 (comment).