updated TIFF reading and imshow bug

TIFF files were black (due to 12-bit encoding) in OpenPIV-Matlab when showing not the first file
the openpiv_imread function is introduced to solve this problem with unifying use of pre-processing image function, in this case imadjust - this probably shall be turned optional, not default?

also the ReadImDir is updated with the option to read lower-case and upper-case extensions at once.