gcmarray internals revamp, skip allocs by default in read and similar

[gaelforget]

• similar no longer allocates InnerArrays by default (likely a breaking change)
  • read! instead does the alloc when needed
  • full_init=false vastly reduces allocs across the package
  • old behavior is currently recovered via full_init keyword arg
  • not yet implemented for the broadcasted version
• getindex, view, and similar were first rewritten, which reduced allocs to start with
• lazy init — copyto! will allocate on first write
• use full_init=true when needed; add missing tests
• rename full_init arg as allocate, update docstrings

[codecov]

Codecov Report

❌ Patch coverage is 84.53608% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.30%. Comparing base (124cc5a) to head (55562ae).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/types/gcmarray.jl	82.95%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
- Coverage   87.50%   87.30%   -0.21%     
==========================================
  Files          37       37              
  Lines        3538     3600      +62     
==========================================
+ Hits         3096     3143      +47     
- Misses        442      457      +15     

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