Fix(tridiagonal_matrices): spmv y vector overwrite and missing error check in impure init functions

[Mahmood-Sinan]

Summary:
This pr fixes two issues within tridiagonal_matrices module.

1. Inside the impure functions for initializing tridiagonal_matrices, the matrix fields were assigned even if there was an error(for example, when the sizes of du and dl vectors differed from dv by more than one).
2. Inside spmv subroutine, the y vector was forcefully being zeroed instead of using the passed argument.

Changes:

1. Added an if (err0%ok()) check in the impure initialization functions.
2. Removed the unintended y = 0.0_${k1}$ in the spmv subroutine.

[loiseaujc]

Thanks for spotting some things I completely missed. Could you also extend the tests to ensure that the cases with $(\alpha, \beta) \neq (1, 0)$ work correctly.

[codecov]

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Additional details and impacted files

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[loiseaujc]

As it is, we do not perform any check in spmv to ensure $(\alpha, \beta) \in (-1, 0, 1)^2$ nor that op = N, T or C. I was wondering whether we should include such checks to dumb-proof the implementation, in particular the $(\alpha, \beta)$ one since this is a quirk from the lapack implementation of *LAGTM differing from *GEMM.

@perazz, @jvdp1 and @jalvesz : What do you think? Should we modify the interface to be something like

subroutine spmv(A, x, y, alpha, beta, op, err)

where err is a linalg_state_type and have the subroutine potentially raise an error if the alpha, beta or op provided by the user are not admissible? Note that *GEMM does not have an info return flag. I'm not entirely sure what it does if transa or transb are not admissible, neither does *LAGTM. Also, the spmv kernels by @jalvesz for sparse matrices do not ensure that op is admissible.