Added a function to reconstruct a symmetric array

[VictorForouhar]

In SOAP, we compute inertia tensors and store its diagonal and a subset of its off-diagonal elements. To compute the eigenvalues, we need convert these flattened representations into the original shape.

This PR adds a function that can be used to do such reconstruction on all of the computed inertia tensors, rather than doing it one tensor at a time.

I have implemented and tested it assuming swiftsimio is used to load SOAP catalogues. It works for 2D and 3D matricies (e.g. projected and 3D inertia tensors).

It keeps track of units. Here is an example test I ran on COLIBRE SOAP catalogues:

soap_catalogue = sw.load('/snap8/scratch/dp004/dc-mcgi1/soap_colibre_tensors/L0012N0376/Thermal_fiducial/SOAP_uncompressed/halo_properties_0123.hdf5')

# Load only those with non-zero entries
stellar_inertia_tensors = soap_catalogue.bound_subhalo.stellar_inertia_tensor
stellar_inertia_tensors = stellar_inertia_tensors[(stellar_inertia_tensors != 0).any(1)]

# Print an example and how many entries we have
print (stellar_inertia_tensors.shape)
print (stellar_inertia_tensors[0])

# Reshape matricies into their correct form
stellar_inertia_tensors = build_matrix(stellar_inertia_tensors)
print (stellar_inertia_tensors.shape)
print (stellar_inertia_tensors[0])

# Compute eigenvalues
eigenvalues = np.linalg.eigvalsh(stellar_inertia_tensors)
print (eigenvalues.shape)
print (eigenvalues[0])

And here is the output:

(294, 6)
[ 7.3760958e-07  1.0794657e-06  1.1297204e-06  1.8591939e-07
  7.1408937e-07 -3.7456874e-07] Mpc**2 (Physical)
(294, 3, 3)
[[ 7.37609582e-07  1.85919390e-07  7.14089367e-07]
 [ 1.85919390e-07  1.07946573e-06 -3.74568742e-07]
 [ 7.14089367e-07 -3.74568742e-07  1.12972043e-06]] Mpc**2 (Physical)
(294, 3)
[5.32662612e-08 1.15860554e-06 1.73492395e-06] Mpc**2