Fixes for issues: 339,340,343
Allow inheritance to support testing of modified classes w/o having to change the original or to allow customer-specific variations
Changed objects:  
  ZCL_EXCEL_READER_2007: Added try-catch block to allow reading excelsheets with textfields ( ignore textfields - just read the rest )
  ZCL_EXCEL_READER_2007: Added support of reading hyperlinks
  ZCL_EXCEL_WRITER_2007: Corrected method CREATE_XL_SHEET_RELS to not look at internal hyperlinks
  ZCL_EXCEL:  Allow inheritance to test changes on subobjects w/o changing original ( needed for issue 343 for class ZCL_EXCEL_WORKSHEET )
  ZCL_EXCEL_WORKSHEET: Allow inheritance, Changed row-dimensions to hashtable, added tabletype for this, changed method GET_ROW_DIMENSION to use hashtable
  ZCL_EXCEL_WRITER_XLSM: Allow inheritance
  ZDEMO_EXCEL_OUTPUTOPT_INCL:  Changed default to "Display", Allow using any writerclass that inherits from ZCL_EXCEL_WRITER_2007
  ZDEMO_EXCEL37:  Allow passthrough of .xlsm files as well.  Allow using any reader or writerclass that inherits from the ZCL_EXCEL_READER_2007 or ZCL_EXCEL_WRITER_2007
  ZDEMO_EXCEL41:  Test inheritance - demo for inherited ZCL_EXCEL or ZCL_EXCEL_WORKSHEET