Implement broadcasting for BioSequences

[TransGirlCodes]

Types of changes

This PR implements the following changes:
(Please tick any or all of the following that are applicable)

• ✨ New feature (A non-breaking change which adds functionality).
• 🐛 Bug fix (A non-breaking change, which fixes an issue).
• 💥 Breaking change (fix or feature that would cause existing functionality to change).

☑️ Checklist

• 🎨 The changes implemented is consistent with the julia style guide.
• 📘 I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• 📘 I have added or updated relevant user and developer manuals/documentation in docs/src/.
• 🆗 There are unit tests that cover the code changes I have made.
• 🆗 The unit tests cover my code changes AND they pass.
• 📝 I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• 🆗 All changes should be compatible with the latest stable version of Julia.
• 💭 I have commented liberally for any complex pieces of internal code.

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[TransGirlCodes]

@jakobnissen Ok so, because broadcasting is a bit complex, I'm going to properly document every choice I make, and everything I've learned so far about the machinery here, as I make commits.

[jakobnissen]

Looks good! I'll follow from the sidelines :)

[jakobnissen]

If you get to understand how broadcasting works, can you perhaps check this method ambiguity out?

Base.Broadcast.BroadcastStyle(s1::BioSequences.PFMBroadcastStyle, s2::Base.Broadcast.BroadcastStyle) in BioSequences at /home/jakob/code/BioSequences.jl/src/search/pwm.jl:76

vs

Base.Broadcast.BroadcastStyle(::S, ::Base.Broadcast.Unknown) where S<:Base.Broadcast.BroadcastStyle in Base.Broadcast at broadcast.jl:133

Is this a runtime error waiting to happen or is it OK?

[TransGirlCodes]

I've cleared this from the v3.0 milestone, we're officially gonna push this back to a version 3.X.

[jakobnissen]

Can you rebase this on master? (no rush though)