Generalize ExponentiallyDecayingCouplingTerms

[jhauschild]

Right now, the ExponentiallyDecayingCouplingTerms add couplings with a uniform decay rate lambda.
It should be fairly straight-forward to allow lambda_ to be a non-uniform array with one value per site of the MPS (unit cell).
This could be used for e.g. a SSH-type model with long range, see https://tenpy.johannes-hauschild.de/viewtopic.php?t=302

Slightly annoying, right now the convention is that the strength is ~ lambda^|i-j|, which includes a lambda on the first site.
We should probably change this to something like prod(i < k < j) lambda[k] , which amounts to lambda^|i-j-1| for uniform couplings.
This might break backwards compatibility with existing code using the ExponentiallyDecayingCouplingTerms. I assume that this would not affect many users, though.