Allow filename in write_mrtrix.m to be string or char array

[lukeje]

Currently, write_mrtrix.m crashes if the output filename is a string:

% char array
write_mrtrix(struct('data',rand(3,3,3)), 'test.mif')
% works without problem

% string
write_mrtrix(struct('data',rand(3,3,3)), "test.mif")
% Gives error: 
% > Array indices must be positive integers or logical values.
% >
% > Error in write_mrtrix (line 134)
% > if strcmp(filename(end-3:end), '.mif')

As the error message indicates, this is because array indexing does not work for strings. This pull request uses the Matlab functions fileparts and fullfile to avoid explicit indexing.

I also took the opportunity to make the error message if the extension is not matched a little more informative.

[jdtournier]

Wow, I had no idea there was a difference between a character array and string in matlab... Guess you learn something new every day!

All looks good and sensible to me.