Use username for VC cache structure

[margaretha]

The virtual corpus cache was intended to cache only system-vc, which is stored as files within a vc-cache directory as follows.

data
|-- vc-cache
    |-- DeReKo-CoRoLa-mapping
    |-- dereko-korap-2020-I
    |-- dereko-korap-2021-I

To support caching for user vc, it would be reasonable to include in the directory tree structure.

data
|-- vc-cache
    |-- system (username) 
        |-- DeReKo-CoRoLa-mapping
        |-- dereko-korap-2020-I
        |-- dereko-korap-2021-I