feat: support for input with mix of mono and multitrack audiostreams

[grusell]

Description

It seems to be reasonably common to store a multitrack (stereo) 'Dolby E' stream together with multiple other mono streams in an MXF file. Previously, encore would fail the job if the input had a mix of mono and multitrack audiostreams, except if the first stream was multitrack. This feat adds some support for inputs with a mix of mono and multitrack streams, where the first stream is a mono stream. Given such an input, encore now uses all mono streams that comes before the first multitrack stream, and ignore all following audiostreams.

Note that the support for mixed multi-/monotrack streams is limited in the sense that it does not allow use of all input streams in such a case, only the monostreams that comes before the first multitrack stream can be used. A more complete solution was deemed to complex at the moment.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Some unit tests has been added
• No integration test was added due to difficulties of finding a suitable test file that could be publicly shared.
• Feat has been tested manually and has been running in production for a few months without problems surfacing.

Checklist:

• I confirm that I wrote and/or have the right to submit the contents of my PR, by agreeing to the Developer Certificate of Origin (see https://github.com/svt/open-source-project-template/blob/master/docs/CONTRIBUTING.adoc[docs/CONTRIBUTING]).
• My code follows the style guidelines of this project
• [X ] I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)