STI channel-loop, new PR due to rebase issues from #118 (#149)

### Changes proposed in this pull request:

- implements a channel-loop based version of STI calculations and
according tests
- STI calculation is split into three functions:
speech_transmission_index_indirect, modulation_transfer_function,
ambient_noise_correction and a small function handling final STI
computations.

- The calculations in this pull request originate from the already
reviewed pull request #118. The original PR could not be successfully
rebased, so this new PR was created. No content changes were made—only
the previously reviewed modifications were carried over.

Author: artur-pa