Created preliminary header file for hpd_int
This lays out the three constructors, as well as some preliminary documentation.  The three constructors are as follows:
    - part based - The decimal is cosntructed from two integers, one representing values to the left of the decimal  place, and the second representing values to the right of the decimal place. This will incorporate protections against values greater than the representable range being input as the value to the right of the decimal place.
    - float based - a float within the representable range will be converted to a hpd_int.  While this will introduce error from the approximation of decimal values that floats make, any operations after this should not have any additonal sources of error.
    - string based - a string representing a decimal value will be converted to an hpd_int.  The values to the left of the decimal place (or comma, to provide support for places where that is standard) will be parsed to an integer, as will the values to the right of the decimal place.

I also added swap files to the gitignore, to avoid commits of temporary files if I commit while a file is open.

Committed by: ajthompson