Developper documentation

We should explain some code:
- general scheme to define dedicated methods depending the usage of PartitionedVector parameters: `Val{true}` and `Val{false}`;
- mention #23 to explain how the partitioned dot product is computed.

Start by reviewing how PartitionedVector parameters use Val{true} and Val{false}, then read issue #23 for the partitioned dot-product explanation. Done means the developer documentation explains the general method-definition scheme and links to or incorporates the computation details from issue #23.

Written by the indexing model from the issue text.