Add utils similar to the existing Generators utils for other asset classes

[scdhulipala]

Definitions of get_index and vcat for Storages, GeneratorStorages and Lines to make extracting of a sub set of regions easier. Currently, get_index and vcat is only defined for Generators.

[GordStephen]

Thanks, I just had a couple of stylistic requests/questions. We probably don't want to advertise/use these functions too widely as they obviously break region mappings if used incorrectly, but if there are situations where they're useful we may as well include them for all resource types.

In new PRAS we should just generally have a more friendly system manipulation API :)

[codecov-commenter]

Codecov Report

Attention: 88 lines in your changes are missing coverage. Please review.

Comparison is base (2ded134) 94.68% compared to head (440b108) 90.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   94.68%   90.36%   -4.32%     
==========================================
  Files          39       39              
  Lines        1843     1931      +88     
==========================================
  Hits         1745     1745              
- Misses         98      186      +88     

Files	Coverage Δ
src/PRASBase/assets.jl	52.43% <1.12%> (-47.57%)	⬇️

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[GordStephen]

Looks good. CodeCov indicates that the Generators version of these are tested while the new versions aren't, but I think that's just because vcat and getindex happen to be used within the package (with Generators in the EFC code), not because we have real tests for these.