src/math organization

[Jason Stover]

To write a GLM procedure (which, unlike a time series procedure, would
be useful to a lot of people), I need to create another data structure
I'll call an "interaction". The interaction, like the code in design-matrix.[ch]
and coefficient.[ch], is just a place for "support" data structures 
for the model-fitting code in directories like math/linreg.

Right now, there is a circular dependency in src/math because the code
in src/math/linreg relies on src/math/design-matrix.[ch] and 
src/math/coefficient.[ch],
but src/math/libpspp_math.a relies on src/math/linreg/libpspp_linreg.a.
Before adding interaction code, I'd like remove this circular dependence.
coefficient.[ch], design-matrix.[ch] and interaction.[ch] don't need to
know about linreg, time series, or any other model-fitting code in a 
subdirectory
of src/math.

So to fix this, should I create a new directory, say, src/math/model_data,
or place that directory elsewhere? 

Also, I'm not sure what this directory should be called. It will
contain data structures that make pspp able to use the model-fitting
code by matching the output from the model-fitting to the names that
users use for things (like variables, values, data sets) to
model-fitting equivalents, like design matrices. Given that, what
would be a good name for this directory?

-Jason