Adds the blob test to examples/HydroTests/BlobTest_3D

This MR adds the infamous blob test from Agertz+2007.

It includes a moving making script, IC script, and a readme.

Here's how various SWIFT schemes perform at high resolution:

blob_256

I could probably work more on this but I think we're at the 80% stage here, and the improvements are required more on the swiftsimio side.

examples/HydroTests/BlobTest_3D/README.md

+Blob Test
+=========
+
+This test is very similar to the one presented in Agertz+ 2007
+(https://ui.adsabs.harvard.edu/abs/2007MNRAS.380..963A/abstract) that tests
+un-seeded fluid mixing.
+
+You can use the makeIC.py script to generate the initial conditions, and
+then the makeMovie.py script to look at the results. Note that this is a very
+dynamic test, and similarly to the kelvin-helmholtz it does _not_ have an explicit
+answer and is best viewed as a movie with more than a grain of salt.
+
+The expected results are:
+
++ For schemes with surface tension terms, the blob stays together
++ For schemes without surface tension terms, the blob breaks up.
+
+This is at much lower resolution than the original test in Agertz+ 2007.
+To change the resolution, you will need to edit the `makeIC.py` file.
+A similar resolution to the GAD\_10M presented in that paper can be gained
+by using `num_on_side=128`.