Matrix types

`LPStandardForm` etc allow you to provide `SparseMatrixCSC` as the matrix. The code then slices this matrix by rows, which is O(num_columns). This slow path shouldn't be allowed.

Start by locating the LPStandardForm definitions and the code path that slices a SparseMatrixCSC by rows. Confirm how matrix types are accepted and identify the existing tests for these forms, if present. Done means SparseMatrixCSC inputs are rejected before the slow row-slicing path can run.

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