Scalar doesn't necessarily support implicit construction from int or
assignment from int.
Here's the error message I got without this fix:
```
/home/tav/git/Sleipnir/build/_deps/eigen3-src/Eigen/src/SparseCore/SparseVector.h:180:25: error: no match for ‘operator=’ (operand types are ‘Eigen::internal::CompressedStorage<ExplicitDouble, int>::Scalar’ {aka ‘ExplicitDouble’} and ‘int’)
180 | m_data.value(p + 1) = 0;
| ~~~~~~~~~~~~~~~~~~~~^~~
```
See merge request libeigen/eigen!2046
(cherry picked from commit 9234883914)
48d2b101 Fix SparseVector::insert(Index) assigning int to Scalar
Co-authored-by: Tyler Veness <calcmogul@gmail.com>
748962722c