Assignment
Write a function
number2pattern two integer $$i$$ and $$k$$.
The function must return the string at index $$i$$ among all
lexicographically ordered DNA k-mers.
Example
>>> number2pattern(45, 4)
'AGTC'
>>> number2pattern(5353, 7)
'CCATGGC'
>>> number2pattern(8799, 8)
'AGAGCCTT'