Cleanup initialization of TimeStepRange/ModeShapeRange.

vtkExodusIIReader now initializes TimeStepRange and ModeShapeRange in
just 1 place consistently i.e. in a method called in
RequestInformation().

Also making the reader not change the values of TimeStepRange and
ModeShapeRange based on whether the HasModeShapes is true/false. This
makes it hard for applications to consistently access this information
since it would change based on the value of HasModeShapes.