Add tryencode and trydecode

[jakobnissen]

Add tryencode and trydecode

The encode and decode methods are not allowed to produce invalid data. Instead,
they throw an error when encountering invalid input data.
This can lead to some frustration when checking if a symbol is permitted in an
alphabet.
One way to solve it is by checking symbol in symbols(A), but this is not
particularly effective.

This PR adds a tryencode and trydecode method to existing alphabets. These
methods return nothing when given invalid data.
Methods encode and decode now internally call their try-variants.

May solve #219

Types of changes

This PR implements the following changes:

• ✨ New feature (A non-breaking change which adds functionality).

📋 Additional detail

• tryencode and trydecode introduced, returning Union{Nothing, E}
• encode and decode now uses a fallback implementation which calls these try- methods internally
• There is no performance difference
• Alphabet docstring has been changed in a backwards compatible way to mention these new methods.

☑️ 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.28%. Comparing base (bf4aac5) to head (4fdfac9).
Report is 63 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   89.25%   89.28%   +0.02%     
==========================================
  Files          31       31              
  Lines        2430     2426       -4     
==========================================
- Hits         2169     2166       -3     
+ Misses        261      260       -1     

Flag	Coverage Δ
unittests	89.28% <100.00%> (+0.02%)	⬆️

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[jakobnissen]

CodeCov is acting up. There is no code coverage difference as far as I can see.
... and now the check magically passed without me pushing anything? What on Earth?

[jakobnissen]

Superseded by #324