Add truncate function for tensors in a Chain

[jofrevalles]

Summary

Add a truncate function that truncates a tensor in a Chain. This function should look into the state, check for singular values, and truncate accordingly.

[mofeing]

Just a note: This truncate function implements local truncation of a virtual index. Whether that local truncation is globally optimal depends on the canonization.

[jofrevalles]

Closing this as it was implemented in PR #16