expv error estimate: hermitian only

[daviehh]

Checklist

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

https://github.com/SciML/ExponentialUtilities.jl/blob/18e5161ad28e49e2b92c5f4879495dbd55a0b52e/src/krylov_phiv_error_estimate.jl#L29

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

Fixes #160

-- whitespace changes is with JuliaFormatter's sciml style.