maint: Refactor general/isXXX.m scripts to put input validation first. * general/isdir.m, general/isequal.m, general/isequalwithequalnans.m, general/isscalar.m, general/issquare.m, general/isvector.m: Refactor code to put input validation first. * general/iscolumn.m, general/isrow.m : Remove useless initialization of output variable. * general/isa.m: Add additional tests for various classes.