Generate a C# enum for every enum the catalog carries

[estebanzimanyi]

MEOS states thirteen enums beside the error codes — interpType, MeosType,
tempSubtype, IndexSearchOp, SPTreeKind, MeosOper, spatialRel,
SkipListType, nullHandleType, MeosPixType, COMPRESSIONS, ENDIANS,
PC_FILTERTYPE — and the catalog carries each with its members and their
values. objectgen.py projects them into MEOS.NET/Enums/, one file each, so
a caller names a MEOS constant where the C API takes an int and the values
come from MEOS rather than from a transcription.

The names are the ones MEOS gives: the type is its catalog name with an
upper-case first letter (interpType reads InterpType, MeosType and
ENDIANS stand), and a member is its constant PascalCased (INTERP_NONE
reads InterpNone, T_TFLOAT reads TTfloat). The generator refuses an enum
whose members would collide into one C# name.

Four tests hold the values to MEOS itself: meostype_name,
interptype_name and tempsubtype_name name back what the member names, and
interptype_from_string reads a name back as its member. A value that drifted
from MEOS names something else, or nothing.

Two files in the library are written by hand, both of them the runtime
substrate: the MEOS session lifecycle, and the error check every wrapper makes
after a call. Everything else under MEOS.NET/ is a projection of the
catalog.