get_nncontext -> init_nncontext and update docs for pip (#327)
get_nncontext -> init_nncontext and update docs for pip (#327)

* update

* modify

* update python docs

* update scala

* meet review

yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL