Make Impact.impact_at_reg support all zero impacts

[peanutfun]

Changes proposed in this PR:

• Detect if the shape product of the impact matrix is zero instead of the nonzero entries. The latter could also be the case for a correctly-shaped matrix whose values are all zero.

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[timschmi95]

The code looks good to me. I'm wondering if there is a use case to calculate imp_at_reg if all the values are zero? If not, one could also throw a different error (or warning) saying that all impacts are zero if self.imp_mat.nnz == 0 .

[peanutfun]

When calibrating impact functions (see #692), one usually aggregates impacts in space, and impact_at_reg is the "canonical" way of doing that. When the optimization algorithm explores possible parameter sets, it might happen that the computed impact is zero everywhere. However, the calibration cannot recover from the error thrown in this case and aborts.

One has to distinguish two cases.

• If save_mat=False, impact_at_reg cannot work, so I think throwing an error is fine. However, there is no way of knowing that for sure from the Impact object. By default, the impact matrix should be empty with shape (0, 0).
• If save_mat=True, it might very well be that all values are zero and computing the spatial aggregate will also return zero. However, this should not be an error (currently, it is), and I also don't see why this should issue a warning. It's simply the arithmetic result.

[timschmi95]

Ah yes, calibration is a good example where all impacts can be zero. And I guess if user has 0 impacts and calculates their 0 impacts for each region it doesn't neccesarily require a warning, although it's not a very interesting calcuculation^^
From my point of view you could merge it then.

[peanutfun]

Thanks for the review and discussion, @timschmi95!