Use smaller Int types where possible

[nickrobinson251]

Part of #9, to reduce memory

On main:

julia> using BenchmarkTools, PowerFlowData
[ Info: Precompiling PowerFlowData [dd99e9e3-7471-40fc-b48d-a10501125371]

julia> str = "raw/realistic30.raw";

julia> @btime parse_network($str);
  160.294 ms (692 allocations: 66.34 MiB)

julia> @btime parse_network($str);
  160.033 ms (692 allocations: 66.34 MiB)

this PR:

julia> @btime parse_network($str);
  151.472 ms (692 allocations: 58.67 MiB)

julia> @btime parse_network($str);
  156.626 ms (692 allocations: 58.67 MiB)

(i'm a little doubtful of the speed change, because we saw the opposite in #7 (comment) (although that was pre-#23), but total memory is reduced as expected).

[codecov-commenter]

Codecov Report

Merging #33 (718b30a) into main (0ea03c5) will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   91.32%   90.96%   -0.37%     
==========================================
  Files           3        3              
  Lines         173      166       -7     
==========================================
- Hits          158      151       -7     
  Misses         15       15              

Impacted Files	Coverage Δ
src/types.jl	96.42% <ø> (ø)
src/parsing.jl	96.96% <0.00%> (-0.21%)	⬇️

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