Change labman.well table to store row_num and col_num as zero-based

Per @antgonza ( https://github.com/jdereus/labman/pull/340#issuecomment-416673419 ):

The Plate.layout property (a list of lists) accesses wells on a plate with zero-based row and column numbers. However, the database (in the labman.well table) stores row and column numbers as 1-based. Since layout's indexing can't really be changed, changing the values stored in the database to be 0-based would save a lot of +1-ing and -1-ing throughout the code.