## Estimate of matrix backend entry numbers is wrong

### Summary

`estimate_mbe_entries`

produces wrong estimates of the matrix backend size.

When constructing the matrix backend, one can given an estimate of the number of matrix entries per element for faster resource allocation. The old values are wrong.

The numbers were extracted from the `patternStatistics`

of the `OneStepGridOperator`

jacobian, but are related to the wrong blocking that was used before the update to DUNE v2.6 (see #68 (closed), !56 (merged))

### Correct values

For a DG method, the numbers are

- simplex:
`dim + 2`

- cube:
`2*dim + 1`

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