Make sure the warnings are raised appropriately

#### Description:
Currently every warning/error event generates a new item in the reflection stack. This results in hundreds of similar warnings being exposed to the user, which makes them tl;dr.

#### How to replicate:
Pull 100 elements with material _By Category_ with GH - only 15 first unknown material warnings will be seen.

#### Expected behaviour:
My idea is as follows: one warning per issue, then optionally supplement the message with a list of ElementIds/BHoM_GUIDs?

@FraserGreenroyd @adecler @al-fisher would it be possible to buffer the duplicate warnings (_almost_ duplicate: same issue but different ElementId or BHoM_GUID) in Reflection and format the warning message on the UI end, or should they be stored on Revit_Toolkit side and formatted before calling `BH.Engine.Reflection.Compute.RecordWarning`?

Self-reminder:
- resolve #820 into account when working on it