Move BH.oM.Adapters.Filing.File object into non-Adapters namespace

As discussed in the PR comment https://github.com/BuroHappoldEngineering/Filing_Toolkit/pull/52#pullrequestreview-491650104
Suggest migration of the `BH.oM.Adapters.Filing.File` into a non-adapters namespace to more clearly denote it as the agnostic BHoM definition of a file. i.e. sits in a oM namespace such as `BH.oM.Filing` without the ~`Adapters`~

This neutral `BH.oM.Filing.File` - is then converted into the SharePoint File or Filing System file as part of the Adapter process.

With this change to namespace for the core File - I might then suggest something like `BH.oM.Adapters.SystemFiling.File` instead of `SFFile` to more clearly denote the file type. Leaning on the namespaces for clarity as per other disciplines.

On the canvas then when creating a "File" - will be logical to use the `BH.oM.Filing.File` as all the others will be in Adapter namespaces, I think

@rwemay @alelom FYI