time handling and merge/concatenation changes

• Fixed bug swapping bytes order for signed int 8bytes with dataRead module.
• Time handling improvement: fileMetadata['time'] epoch floating in seconds is commonised for all mdf versions, no more dates as found in mdf version 3. More versatile this way and python datetime module can still be used to extract date if needed.
• Reworked merge_mdf to cope with heterogeneous data and renamed into concat_mdf
• Created merge_mdf