[ENH] Accept more input images formats in SlicerASLtoolkit modules

The modules (CBF and MultiTE) receives the input imagens only in .nii and .nii.gz formats.
This is set in XML file and make the path selector filter the inputs to the user. 

However, it could be better to include other formats as well (e.g .nrrd, .nhdr, etc), which will facilitate to user select compatible images without changing the filter selector