Move workspace file based operations to ISaveable class and simplify file-based operations

[abuts]

Current structure of fileBased event workspace is very complex. The file based/ not-file based operations are controlled by five boolean parameters, and user has to set all of them in correct order to get working file-based algorithm.

The Kernel::DiskBuffer tests introduces some algorithm to work with file based workspaces and these algorithms are repeated within different MDAlgorithms.

There are two independent modes of working with file based workspaces and it is unclear which one to use.

The purpose of this ticket is to simplify the construction of file-based MDWorkspaces, move main file-related functionality into ISaveAble DiskBuffer couple of objects and simplify MDBoxes/MDGridBoxes removing this functionality from them.

[abuts]

This issue was originally trac ticket 6362