Cannot sandwich SplitMatrix with non-owned array

[david-cortes]

This throws an error:

import numpy as np
import tabmat
from scipy.sparse import csc_matrix

rng = np.random.default_rng(seed=123)
X = rng.standard_normal(size=(100,20))
Xd = tabmat.DenseMatrix(X[:,:10])
Xs = tabmat.SparseMatrix(csc_matrix(X[:,10:]))
Xm = tabmat.SplitMatrix([Xd, Xs])
Xm.sandwich(np.ones(X.shape[0]))

---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-2-91ba52e4f568> in <module>
      8 Xs = tabmat.SparseMatrix(csc_matrix(X[:,10:]))
      9 Xm = tabmat.SplitMatrix([Xd, Xs])
---> 10 Xm.sandwich(np.ones(X.shape[0]))

~/anaconda3/envs/py39/lib/python3.9/site-packages/tabmat/split_matrix.py in sandwich(self, d, rows, cols)
    287             idx_i = subset_cols_indices[i]
    288             mat_i = self.matrices[i]
--> 289             res = mat_i.sandwich(d, rows, subset_cols[i])
    290             if isinstance(res, sps.dia_matrix):
    291                 out[(idx_i, idx_i)] += np.squeeze(res.data)

~/anaconda3/envs/py39/lib/python3.9/site-packages/tabmat/dense_matrix.py in sandwich(self, d, rows, cols)
     62         d = np.asarray(d)
     63         rows, cols = setup_restrictions(self.shape, rows, cols)
---> 64         return dense_sandwich(self, d, rows, cols)
     65 
     66     def _cross_sandwich(

src/tabmat/ext/dense.pyx in tabmat.ext.dense.dense_sandwich()

Exception: 

Compare against this:

Xd = tabmat.DenseMatrix(X[:,:10].copy())
Xs = tabmat.SparseMatrix(csc_matrix(X[:,10:]))
Xm = tabmat.SplitMatrix([Xd, Xs])
Xm.sandwich(np.ones(X.shape[0]))

(No error)