Fix beach pdf output when Axes instance is provided to plot into

[megies]

What does this PR do?

This commit is a bug fix, which corrects a bug in the obspy/imaging/…beachball functions.

Specifically, when calling the functions beach(...) in obspy/imaging/mopad_wrapper.py or obspy/imaging/beachball.py with the arguments axes not equals None, there is always an error if save the plotted beachballs into vector files (e.g., .pdf or .eps).

To locate the error/bug, I found that the functions beach(...) calls the function matplotlib.transforms.IdentityTransform() to create a Transform object. However the created object does not have the method translate(...) that is essential for saving plots into vector files.

To fix the bug, it is simple by using the function matplotlib.transforms.Affine2D(np.identity(3)) instead of matplotlib.transforms.IdentityTransform() to create the Transform object. The object created by the former has the method translate(...) and hence allows for saving into vector files, while the object created by the latter does not. Excep this difference, the objects created by the two functions have the same behaviours.

Why was it initiated? Any relevant Issues?

fixes #2887

PR Checklist

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[megies]

Closing for a new PR where we can add commits with original commit cherry-picked.