This commit separates the circuit definition (resistors, op-amps, etc.) from the
solver and matrix libraries. Now, in theory, the circuit definition can be used
not just for small signal AC analysis (as was previously available), but also
for any other linear circuit analysis. Furthermore, the matrix library has been
split into its own module - `solve` - to allow for other libraries to be used
in the future. The matrix library preference is controlled via the configuration
file.