expv error estimate: hermitian only

[daviehh]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Currently, krylov_phiv_error_estimate.jl only implements the Lanczos algorithm for Hermitian matrix, which gives a tridiagonal H to use with the expT! functiopns's stegr call

ExponentialUtilities.jl/src/krylov_phiv_error_estimate.jl

Line 29 in 18e5161

LAPACK.stegr!(α, β, cache.sw)

This PR passes on the ishermitian argument and errors if false.

Fixes #160

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whitespace changes is with JuliaFormatter's sciml style.