Refactor how to create DataAvailabilityHeaders (celestiaorg#524)

* use [][]byte instead of interval digests

* add extra DAH methods

* move the dataavailabilityheader from types to da

* refactor creating DAHs

* finish refactor and clean up

* add tests

* linter

* typos causing tests to fail

* fix docs

* remove test used while debugging

* add docs to indicate that computing the shares also padds them

* update to the latest version of nmt

* change version of checkout to hopefully fix the proto lint ci test

* update hashes to account for new verison of nmt

.github/workflows/proto.yml

@@ -10,13 +10,13 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 4
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2.3.4
       - name: lint
         run: make proto-lint
   proto-breakage:
     runs-on: ubuntu-latest
     timeout-minutes: 4
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2.3.4
       - name: check-breakage
         run: make proto-check-breaking-ci