fix: add exception in Quaternion.average

[MarcoMiretti]

When trying to average an empty list of quaternions, rise a
QuaternionError, explaining that it takes at least one quaternion to
calculate the average.

[coveralls]

Coverage increased (+0.01%) to 98.479% when pulling 6a6fae3 on MarcoMiretti:fix-add-exception-when-averaging-empty-list into 649d47b on satellogic:master.

[matiasg]

Nice!
Just one comment on the test, but +1 from me.

[matiasg]

Why this exception in particular?
Could it be just except Exception?

[MarcoMiretti]

Yes, i guess that's better.

Just to explain why i met this particular exception:
np.linalg.LinAlgError is what happens when we try to np.linalg.eigh(matrix) for a matrix of rank less than two.

In the case of an empty quaternion list gives us:

b = np.array([])
weights = np.array([])

then:

m = 0.0

So we end up calling:

cls._first_eigenvector(0.0)

which leads to:

np.linalg.eigh(0.0)
--> np.linalg.LinAlgError

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In this test, except Exception is much better, since for example, _first_eigenvector could change in the future, checking the rank of its input and raising exceptions if they aren't met. That would break this test unnecessarily.