Add IsNull() to class CAutoFile and remove operator !
CAutoFile: Explicit Get() and remove unused methods

Also add documentation to some methods.
Merge pull request #5108

a873823 CAutoFile: Explicit Get() and remove unused methods (Wladimir J. van der Laan)
fef24ca Add IsNull() to class CAutoFile and remove operator ! (Ruben Dario Ponticeli)