Updated some incompatibilities

[rmustakos]

Handled the case where the filename is not passed into mha_read_volume, which caused an error in calling mha_read_header with an undeclared variable (filename)
fopen doesn't seem to need the filename to be transposed (and Octave hates it)
if fopen doesn't work with big/little endian cues, open it and hope for the best
Functions require an "end" before the next function definition
Added *.mhd to file types to open
Works the same for both Matlab and Octave

Fixes are attached

mha_read_volume.zip

[lassoan]

Thank you for these updates. Could you please send a pull request with the suggested changes to allow a quick review and proper attribution of these updates to your account?