Change the default font size of FSL GUI

FSL is a powerful tool for brain MRI analysis. When using a high resolution display, the default font size of its GUI could be too small (especially on laptop). Here is the way to change that:

1. open the fslstart.tcl file in {FSL_DIR}/tcl/.
2. change the default font size from 10 to 18 points in Line 23 & 24:
    option add *Font {Helvetica 18} widgetDefault
    option add *NoteBook.font {Helvetica 18 bold}
3. save the file.