Refactor bfGetReader input check

- First check for fake string
- Check for file existence using fileattrib()
- Remove unnecessary nargin==0 check

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)
Precise condition of lurawavelicense check

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)