Reader Refact & INCOMPATIBLE remove CAN_READ_FILE

1. Removed interface method ZOF_EXCEL_READER~CAN_READ_FILE
As was suggested in the comments, the method ZIF_EXCEL_READER~CAN_READ_FILE is superfluous, as the exception ZCX_EXCEL in LOAD( ) / LOAD_FILE( ) already gives this information. 

This is an incompatible change! But it's unlikely that this method is used by many clients, as it didn't to anything useful. Also, the demo programs don't use it.

2. Switched the zip archive reader creation from the former "on demand" creation to a one-time creation in the LOAD method. Necessary for the next point:

3. Removed private attribute EXCEL2007 (which kept the input XSTRING): After generation of the zip archive, these data are not necessary any more.

4. Factored out Stefan Schmöcker's "alternate zip" concept into a local subclass: The object "zip" refers to a local class - which is either a proxy to CL_ABAP_ZIP, or creates a zip object of dynamic type and calls its GET method of a class dynamically (the "alternate zip" case).

Depending on the answer of my question 

https://github.com/ivanfemia/abap2xlsx/issues/232#issuecomment-42115340

this abstraction could be removed in a forthcoming commit as soon as it has been clarified that CL_ABAP_ZIP works as expected in all cases.

5. Further cleaning.- Removed some comments like "ToDo: Comment this" if necessary, and also some misleading comments which gave unwanted attention to a certain defect (issue 234) and the circumstances of that defect, thereby obfuscating the far more general nature of the commented code.