Codechange: Replace GroupStatistics' num_engines with std::map. (#11849)

This removes manual memory management with calloc/free calls, and prevents potentially large arrays being allocated for each group.

Codechange: Split CT_INVALID into CT_INVALID and INVALID_CARGO.

INVALID_CARGO is a CargoID and should be used for most purposes in game.
CT_INVALID is a CargoType used for defining default properties.

Codechange: Use range-for to simplify getting first non-zero value.

This reduces indexed array access.

Codechange: Add base() method to StrongType to allow access to the base type without casting. (#11445)

This removes the ability to explicitly cast to the base type, but the requirement
to use .base() means the conversion is still explicit.

Change: Units-system can convert from N to kN, don't preconvert.

This allows force to passed as is and avoid premature rounding.

The AI function "GetMaxTractiveEffort" still needs to return kN to avoid breaking the API.

Add: precondition checks to functions that work with both valid company and deity

These are functions that either use ScriptObject::Command or ScriptObject::GetCompany.
This is a bit over-protective, but having the check everywhere makes it easier to
validate that no check is missing automatically instead of by review.

At this moment these checks will not do anything useful, as either IsValid or
IsDeity from ScriptCompanyMode returns true, but that will change later.

Codechange: use GSCompanyMode::IsValid, IsDeity, and the precondition helpers

Direct 1:1 replacements in the code, and comments now refer to either
GSCompanyMode::IsValid or GSCompanyMode::IsDeity instead of several variations
on "company mode active" or "no company mode active".

Feature: GS method to control engine availability for a specific company (#7791)

* Feature: GS method to allow company to use an engine before its introduction date

* Feature: GS method to retire an engine early for a specific company