ADD: GasLib parsing functions and implementation of pressure reducing valves

[tasseff]

This is a first shot at implementing a GasLib parser. It also includes the implementation of pressure reducing valves, which appear as special resistors in the new GasLib-4197 case. The parser enables the solution of all GasLib instances on the official website when using the gf problem specification with the MISOCPGasModel formulation. I haven't yet tested with other problem specifications or formulations. A review would be appreciated!

[codecov]

Codecov Report

Merging #202 into direction_refactor will increase coverage by 1.42%.
The diff coverage is 77.25%.

@@                  Coverage Diff                   @@
##           direction_refactor     #202      +/-   ##
======================================================
+ Coverage               73.73%   75.16%   +1.42%     
======================================================
  Files                      34       35       +1     
  Lines                    3449     3901     +452     
======================================================
+ Hits                     2543     2932     +389     
- Misses                    906      969      +63     

Impacted Files	Coverage Δ
src/GasModels.jl	25.00% <ø> (ø)
src/core/objective.jl	61.53% <ø> (ø)
src/form/lrdwp.jl	34.14% <0.00%> (-1.76%)	⬇️
src/form/lrwp.jl	43.22% <0.00%> (-0.57%)	⬇️
src/core/constraint_mi.jl	57.47% <37.50%> (-0.83%)	⬇️
src/core/constraint_template_mi.jl	64.33% <50.00%> (+0.21%)	⬆️
src/prob/ls.jl	73.17% <50.00%> (-2.51%)	⬇️
src/prob/ne.jl	68.96% <50.00%> (-1.41%)	⬇️
src/prob/nels.jl	67.24% <50.00%> (-1.28%)	⬇️
src/prob/ogf.jl	66.66% <66.66%> (-1.63%)	⬇️
... and 16 more

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