Pre-relase 0.8: writer support.

- new abstract base classes for header and container
- access to more blocks and packets properties
- track block offset within the cleartext data
- iterator for only deciphered blocks
- abstract container writer
- tests for writer and related functionality

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "oarepo_c4gh"
-version = "0.7"
+version = "0.8"
 authors = [
   { name = "Dominik Pantůček", email = "[email protected]" }
 ]