Extra TelescopeParameter use cases

[watsonjj]

This PR ensures TelescopeParameter:

• Can be correctly set via the args of a Component (correctly setting up TelescopeParameterLookup._subarray_global_value)
• Retains subarray description when the TelescopeParameter defaults are changed

[codecov]

Codecov Report

Merging #1175 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1175      +/-   ##
==========================================
+ Coverage   86.03%   86.06%   +0.03%     
==========================================
  Files         182      182              
  Lines       11502    11534      +32     
==========================================
+ Hits         9896     9927      +31     
- Misses       1606     1607       +1

Impacted Files	Coverage Δ
ctapipe/core/traits.py	97.65% <100%> (-0.7%)	⬇️
ctapipe/core/tests/test_traits.py	100% <100%> (ø)	⬆️

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