feat(measure/bochner_integration): dominated convergence theorem (#1757)
* feat(measure/bochner_integration): dominated convergence theorem
This PR
* proves the dominated convergence theorem
* and some other lemmas including `integral_congr_ae`, `norm_integral_le_lintegral_norm`.
* adds several equivalent definitions of the predicate `integrable` and shortens some proofs.
* fix linting error
* Add some section doc strings
* Indentation is very wrong
* Remove useless assumptions; fix doc strings
* remove `private`; add a doc string for Lebesgue's dominated convergence theorem
* Update basic.lean