Add 'depth' parameter to limit the actual depth of circuit.

[95-martin-orion]

Currently, specifying 'depth' to qFlex instead of using the auto-computed depth will result in an error, since it doesn't actually limit the number of cycles processed from the circuit file.

This will probably be simplest to fix after #115 is completed - until then, documenting this issue in the help message should be enough.

[s-mandra]

Agreed. Let me change the title to be more informative.

[benjaminvillalonga]

See #115

[s-mandra]

At the moment, users cannot specify the depth so I declassify this as improvement rather than bug.