generalize contraction code

[95-martin-orion]

In order for qflex to support arbitrary circuit layouts, we need to write a generic version of the contraction code which can take contraction details (i.e., ordering and cuts) as input.

[95-martin-orion]

On a similar note, we should be able to take the "qubits_off" grid as input as well - either as a bitstring or a boolean array.

[95-martin-orion]

PR #27 will resolve this, more or less. There's still room for changes to how users provide input, but that can be tracked in a separate issue.

[95-martin-orion]

As of #30 this is complete - all circuit details can be provided through config files, and the main binary is completely generic.