Groundstate bug in QuTiP 5

[christian512]

Bug Description

The groundstate() function seems to have a bug in the current version of dev.major.
When calling groundstate() on any Qobj, an error occurs.

Code to Reproduce the Bug

from qutip import destroy
H = destroy(2).dag() * destroy(2)
H.groundstate()

Code Output

File "/home/chris/git/qutip-dev-major/qutip/core/qobj.py", line 1632, in groundstate
    grndvec = Qobj(evecs[0], dims=new_dims)
TypeError: 'qutip.core.data.dense.Dense' object is not subscriptable

Expected Behaviour

The groundstate of H should be returned.

Your Environment

QuTiP Version:      5.0.0.dev0+98a8e7d
Numpy Version:      1.22.4
Scipy Version:      1.8.1
Cython Version:     0.29.30
Matplotlib Version: 3.5.2
Python Version:     3.8.10
Number of CPUs:     4
BLAS Info:          OPENBLAS
INTEL MKL Ext:      False
Platform Info:      Linux (x86_64)

Additional Context

No response