Structure_oM: `BarPointLoad` requires a new `bool` property to clarify whether `DistanceFromA` property is parametric or absolute

#### What is missing/incorrect?

I had this question asked from an external collaborator -- we need to specify whether the distance is intended as parametric or actual in the attribute description:

https://github.com/BHoM/BHoM/blob/95d6557547f2bedb1c5262cb06772be67781244f/Structure_oM/Loads/BarPointLoad.cs#L39-L40

~~After confirming with @peterjamesnugent , adapters assume that this is parametric (as it is also for other loads, [e.g. `BarDifferentialTemperatureLoad.TemperatureProfile`](https://github.com/BHoM/BHoM/blob/95d6557547f2bedb1c5262cb06772be67781244f/Structure_oM/Loads/BarDifferentialTemperatureLoad.cs#L43-L44)).~~ After chatting with @IsakNaslundBh this is assumed as absolute distance in other places, and the general idea is that we do want to retain flexibility for convenience reasons, possibly adding an Engine method to convert between the two.

Therefore, we need to add another `boolean` property to this object that states whether the distance is to be interpreted as absolute or parametric. I am converting this issue from `documentation` to `feature`.

Start with Structure_oM/Loads/BarPointLoad.cs at the linked lines, then compare the distance semantics with BarDifferentialTemperatureLoad.TemperatureProfile. Check how BarPointLoad is consumed elsewhere before deciding the property’s interpretation and default behavior. Done means the object can explicitly distinguish absolute from parametric distance and its affected usages remain consistent.

Written by the indexing model from the issue text.