added open_data_file_year() function for direct file opening

[answerquest]

Usage demo on: https://gist.github.com/answerquest/16aac80eab154fd276cabcf4eafbcb33

example for syntax:

data = imdlib.open_data_file_year('tmax','Maxtemp_MaxT_1979.GRD',1979)

1st arg: var_type : 'rain', 'tmax' or 'tmin' 2nd arg: filename / path to file 3rd arg: year for which data is

[answerquest]

Solves #18