An interface that can actually be called

[eggrobin]

The ExternalInterface abstracts away the fetching and passing of the plugin, and converts statuses into more idiomatic exceptions; it turns out parameters into return values, since the former are annoying to use by reflection.
By wrapping the Interface.ExternalMeow calls, it works around the calling convention issues described below.

See the draft documentation at https://github.com/mockingbirdnest/Principia/wiki/Interface-for-other-KSP-mods.

The API added in #2079 for #2074 is cumbersome to call directly (the caller has to look for a PrincipiaPluginAdapter, fetch its private plugin_ field, and check that the private static field Loader.loaded_principia_dll_ is true), and calls to functions with parameters fail hard anyway, because mono somehow messes up the calling convention when calling a P/Invoke function directly by reflection: an attempt to call

 internal static extern Status ExternalGeopotentialGetCoefficient(
     this IntPtr plugin,
     int body_index,
     int degree,
     int order,
     out XY coefficient);

extern "C" PRINCIPIA_DLL
Status CDECL principia__ExternalGeopotentialGetCoefficient(
    Plugin const* const plugin,
    int const body_index,
    int const degree,
    int const order,
    XY* const coefficient);

with

plugin = 0x5AFEA730,
body_index = 1,
degree = 2,
order = 0

led to, on the C++ side,

plugin = 1,
body_index = 2,
order = 0x3F3B58D0
coefficient = 18

and a subsequent access violation when dereferencing 1.

[pleroy]

retest this please