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

[al-fisher]

As discussed in the PR comment #52 (review)
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