Add a TypedReadOnly trait

[itziakos]

This PR adds a Typed version of the ReadOnly trait.

[itziakos]

sorry @corranwebster and @mdickinson I thought that I had the PR ready but it looks I am still missing something

[itziakos]

@mdickinson and @corranwebster this pr is ready for review.

[codecov-io]

Codecov Report

Merging #488 into master will increase coverage by 0.33%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   65.15%   65.48%   +0.33%     
==========================================
  Files          44       44              
  Lines        7040     7061      +21     
  Branches     1413     1416       +3     
==========================================
+ Hits         4587     4624      +37     
+ Misses       2030     2014      -16     
  Partials      423      423

Impacted Files	Coverage Δ
traits/api.py	86.2% <ø> (ø)	:arrow_up:
traits/trait_types.py	63.98% <100%> (+0.58%)	:arrow_up:
traits/has_traits.py	66.32% <0%> (+0.07%)	:arrow_up:
traits/trait_handlers.py	63.4% <0%> (+0.33%)	:arrow_up:
traits/etsconfig/etsconfig.py	63.58% <0%> (+6.17%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update c602b70...94cd6bb. Read the comment docs.

[mdickinson]

@itziakos Thank you! I'll look at this within the next couple of days.

[corranwebster]

I'm OK with the behaviour of returning a default before a value is set - I think that might be a more pragmatically useful behaviour - but perhaps the name of the trait is wrong: should it be SetOnce or something like that?