Allows the user to define distances between different bases.
E.g. An A-T should get a smaller difference
as an A-G, so the corresponding matrix elements should be set
to 1.0 and 2.0.
NOTES
Before the algorithm starts, all matrix values will be
multiplied by a factor, so that the average matrix value
(except the diagonal) gets a value of one !!!
To save a user matrix use <Properties/Save Properties> menu
item.