Implement in terms of TLazyDS a lazy cache

Presently cache is an instant "operation". What one could do is to make it lazy via the TLazyDS.
One could re-implement it as follow:

• One "takes" the selected columns from the TDF
• Initialises a TLazyDS with the TResultProxy<vector<T>> instances obtained
• Initialises a TDF with that DS and return it

Upon utilisation of the cached TDF, the caching will be lazily triggered.
*This functionality implies an interface change*: the Cache method would return a TResultProxy<TDataFrame> instance rather than a TDataFrame proper ("->" rather than "." to be invoked, at the end of the day...)