Change arguments type of SplitMemoryLinks from IResizableDirectMemory to String

Current implementation:
```C#
SplitMemoryLinks(IResizableDirectMemory dataMemory, IResizableDirectMemory indexMemory)
```
Next implementation:
```C#
SplitMemoryLinks(string dataMemory, string indexMemory)
```

Locate the SplitMemoryLinks entry point and all callers in the Data.Doublets codebase. Change the two arguments from IResizableDirectMemory to String, then build the library and run the available tests to confirm the updated signature is used consistently.

Written by the indexing model from the issue text.