added doctests for compare_string and is_for_table

[pathak-deep15]

compare_string('0010','0110')
'0_10'
is_for_table('__1','011',2)
True
The above doctests were added

[cclauss]

This is awesome and it works as expected but instead of putting all the tests into a single comment, I would encourage you to move the doctests under their appropriate functions. The reasons for this are code reuse and readability. Let's say that I read this file and I would like to copy the algorithm compare_string() and paste it into my own project. It would be good if the testcase moved along with the function. The readability dimension comes in when I am reading through the code, I see the definition of the function and immediately after I see a sample call which helps me to know if I properly understand the intention and effects of calling that function.

Great work and thanks for your contribution.