Matrices - create a lowmem class, factor out common methods

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The Biodiverse::Matrix structure has a large memory overhead, using ~3.3-5 
times as much memory as a plain two level hash (which it uses as a 
substructure).

This can cause the system to exceed the memory requirements.  

A number of indices also depend on a matrix of values as a precalc or cache, 
but don't need the methods the overheads are there to support/accelerate.  

A low memory matrix class could also be used as a drop-in replacement elsewhere 
if one is willing to sacrifice some speed for less memory consumption.

Original issue reported on code.google.com by shawnlaffan on 15 Aug 2012 at 11:42