add separate compilation for feature engineering

[Sprate]

Thanks for your comments #238 ! @honshj The main changes are as follows:

1. I create a new package for the feature engineering project that doesn't rely on paddlefl, support separate compilation and pip install. The pip version along with paddlefl will be supported sooner.
2. the C++ codes realatd to paillier are put under 'PaddleFl/core/he'.
3. the python code and proto are put under 'PaddleFl/python/feature_engineering'.
4. revised some expressions and code specification.
   Thanks for your review and help! @honshj @kaih70

[kaih70]

LGTM

[honshj]

can we do this with cmake?

[honshj]

如果能在cmake里面完成protoc，那么这个步骤可以省略

[Sprate]

1.在cmake里没有编译c++ 的grpc;直接使用了python的grpc;
2.proto生成的metrics_pb2_grpc.py 必须是 from paddle_fl.feature_engineering.proto import metrics_pb , 或者 from . import metrics_pb2 ; 自动生成的前面一般没有这个from xxx，那么在pip install 时候会报错“no modulo named xxx”
综合考虑在这里使用手动生成的方法

[Sprate]

@honshj 在最新的提交中，增加了cmake 编译grpc proto的方式，在单独编译时需要先编译protobuf和grpc。

[honshj]

LGTM