add md5 sum check for Eigen3 download via Install.py

version = '3.3.7'

tarball = "eigen.tar.gz"

# known checksums for different Eigen versions. used to validate the download.

checksums = { \

              '3.3.4' : '1a47e78efe365a97de0c022d127607c3', \

              '3.3.5' : 'ee48cafede2f51fe33984ff5c9f48026', \

              '3.3.6' : 'd1be14064b50310b0eb2b49e402c64d7', \

              '3.3.7' : 'f2a417d083fe8ca4b8ed2bc613d20f07' \

}

# help message

help = """