Implement battery item type

[ymber]

Summary

SUMMARY: Infrastructure "Implement new item type for batteries"

Purpose of change

#32067

Describe the solution

• Allow loading units::energy from json
• Implement BATTERY item type
• Allow recharging BATTERY type batteries
• Update item::info for battery islot data
• Support saving and loading current battery charge
• Document new JSON

[ifreund]

Looking great, will be super nice to have everything using proper energy units in the code.

[ifreund]

Not sure if you saw this already, but travis is failing because of an actual problem compiling on gcc 5.3

src/savegame_json.cpp:1926:47: error: specialization of ‘template<class V, class U> void units::quantity<V, U>::deserialize(JsonIn&)’ in different namespace [-fpermissive]
 void units::energy::deserialize( JsonIn &jsin )
                                               ^
In file included from src/cata_utility.h:13:0,
                 from src/player.h:20,
                 from src/avatar.h:12,
                 from src/savegame_json.cpp:29:
src/units.h:134:14: error:   from definition of ‘template<class V, class U> void units::quantity<V, U>::deserialize(JsonIn&)’ [-fpermissive]
         void deserialize( JsonIn &jsin );
              ^

[ymber]

I saw it but I didn't know how I ought to handle it because it's a compiler bug. It got fixed in GCC 7.0. How far back do we need to support old compilers?
GCC bug 56480

[kevingranade]

See https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/COMPILER_SUPPORT.md
Not at all coincidentally, the minimum compiler version we test in Travis is the minimum complier version we support.

[kevingranade]

Can you simply define the specialization in the namespace? I.e. wrap lines 1925-1929 in namespace units {}?