Make signatures initialisable with perry and marusic defaults

[thclark]

From the graphs in Perry and Marusic 1995, we can digitise the approximate signatures originally used in the paper. This is useful for validation purposes. They also do not need to be computed extensively, so we can use them as basic defaults.

[thclark]

Partly achieved - see the test cases for digitised signatures which could be interpolated to the spacing we require