Added ability to add and subtract listingtable objects
Merge branch 'master' into listingtablediff
Added documentation of addition and subtraction of listingtable objects
Reformatting docstring in t2listing get_difference() method
Bugfix in fortran_float(): would return NaN for blank strings- now returns zero
Further revision to fortran_float() function- could fail for negative numbers with missing E exponent signifier
Further revision to fortran_float() function- could fail for negative numbers with missing E exponent signifier