Uniform warnings and errors in intializers

[MartinuzziFrancesco]

Currently the errors for certain issues on the inits are not standardized. All the reservoir initializers should have

• Error if sparse is called but SparseArrays not in scope (already implemented)
• Error if the matrix is not square
• Check on the values if the spectral radius is rescaled
• Additional, init specific warnings/errors

[MartinuzziFrancesco]

Connected to this: throw a warning for weighted_init when the reservoir size is changed (ref). This should help users when they get an error in the ESN that the dimensions are not matching