Creating a n-dimensional state vector space for nonperfect square n

[nkoskelo]

I am opening this issue now, I will fix it next week.

On version '0.9.13.1', I am getting an error when I try to make a basis for a two dimensional state vector space.

from pygsti.baseobjs.basis import BuiltinBasis
b = BuiltinBasis("sv", 2)

Suggests that the dimension needs to be a perfect square, but one should be able to make a state vector of any dimension.

Here is the error message.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/name/pg-misc/pygsti-repo/pygsti/baseobjs/basis.py", line 1071, in __init__
    else _statespace.default_space_for_dim(dim_or_statespace)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/name/pg-misc/pygsti-repo/pygsti/baseobjs/statespace.py", line 1296, in default_space_for_dim
    assert(udim**2 == dim), "`dim` must be a perfect square: %d is not" % dim
           ^^^^^^^^^^^^^^
AssertionError: `dim` must be a perfect square: 2 is not

For a four dimensional state vector space we see that,

print(BuiltinBasis("sv", 4))

returns

State-vector basis (dim=4), 4 elements of shape (4,) :
|0>, |1>, |2>, |3>