modified plot
added init method of reader
got to know latlon limits
added names for lat and lon in different netCDF file readers
added the possibility of getting lat and lon within desired limits (needed for imposing limits on values read)
added translation of lat and lon to jx and iy coordinates for CRU reader
added the possibility of reading values from the reader
added the possibility to create empty value (for later updating from different datasets)
added the possibility to update the value (with data from another value)
to_c and to_K methods moved from Temperature back to Value (not a really good solution, needs refactoring)