Make a function for splitting a concatenated matrix

[gaballench]

Quite often the issue is that tree inference programmes allow to use a partition block based on positions from a continuous MSA, often with dynamic settings for e.g., codon-position:

partition1 = 1-999 /3, 2-999/3
partition2 = 3-999 /3

Most tools I could find just take start:end specifications and produce independent blocks using that information. However, these are the simplest case and complex partition schemes are often available when we want e.g. tree inference with raxml or iqtree and then divtime inference with mcmctree which only takes input in phylip format.

We may want to attempt to implement something targeting these situations.