System abbreviation error output

[michihdeu]

the errors given when two lines in systems.csv have the same system abbreviation are not very helpful at diagnosing the problem

TravelMapping/RailwayData#140 (comment) by @neroute2

[yakra]

Tried it out for myself and I see what you mean. 😬

The solution is easy enough, just use an unordered_set to make sure system codes are unique when reading systems.csv.
Though if I'm doing that, may as well make a static unordered_map member of the HighwaySystem class, similar to Region::code_hash. That will help clean up the routines that parse systemgraphs.csv, multisystem.csv & fullcustom.csv, thus tying in with #614, but I don't expect it to get much use other than that.

Similar stuff we could also do (though is it worth the bother?):

• A similar check for region codes in regions.csv (which doesn't get edited all that much)
• A check for duplicate full names. Could be useful if a systems.csv line is copied & pasted, and then not fully edited.

[yakra]

Upcoming pull request will check for duplicate system abbreviations and report those, rather than the flood of unhelpful error messages @neroute2 noted.

It will not add either of the other checks I considered above.