Check sparsity in LTIModel.from_* methods

[pmli]

For example, the clamped beam model from the SLICOT benchmark has the A matrix of size 348x348 stored in a sparse format, but more than 50% of the entries are nonzero (which makes computations much slower than if a dense format was used). It might be good to check the sparsity and automatically decide on a storage format.