PyPE solution to PE problem 81

    ProjectEuler.net problem 81
    ===========================

    In the 5 by 5 matrix below, the minimal path sum from the top left to the
    bottom right, by only moving to the right and down, is indicated in bold
    red and is equal to 2427.

        131 673 234 103 18
        201 96 342 965 150
        630 803 746 422 111
        537 699 497 121 956
        805 732 524 37 331

    Find the minimal path sum, in matrix.txt (right click and 'Save Link/Target
    As...'), a 31K text file containing a 80 by 80 matrix, from the top left to
    the bottom right by only moving right and down.

    Solution
    --------

    We begin at the apex (0,0) and work our way down and right, recursively
    traversing the grid. Each time we calculate the maximum sum at a given
    co-ordinate, we cache the value for future use. This is very similar to
    the solution used for problem 18, only with a grid instead of a triangle.

    Since we are looking for a minimum, we cannot instantiate our search results
    to 0; we must instead instantiate them to a number larger than we will ever
    see. We call this `big_s` (for big-sum). If big_s is not given, it is taken
    to be the maximum value in the grid multiplied by the area of the grid.

    Answer
    ------

    427337