Macro for creating map structs

[tsmbland]

Description

Written a macro to create map structs, and applied this to DemandMap and CommodityCostMap

Planning to keep using this macro to implement all the necessary map structs, as discussed in #476, so it may be refined/improved further as I go along

Fixes # (issue)

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.03%. Comparing base (043e63e) to head (dcdfdd9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #489   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files          36       36           
  Lines        4893     4893           
  Branches     4893     4893           
=======================================
  Hits         4650     4650           
  Misses        123      123           
  Partials      120      120           

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[tsmbland]

I've made an alternative PR (#490) which is definitely simpler. Interested to hear thoughts on both

[tsmbland]

Going with #490 instead