Change naming of Class and modify imports for src modules

[antaenc]

For all engine modules:

From:

from senzing import SzEngine as SzEngineAbstract
from senzing import SzEngineFlags

To:

from senzing import SzEngine, SzEngineFlags

From:

class SzEngineCore(SzEngineAbstract):

To:

class SzEngineCore(SzEngine)