Add 'activations' to GraphSAGE and HinSAGE classes

[adocherty]

Description

Currently the GCN and GAT layers allow specifying the activations at each layer of the network. GraphSAGE and HinSAGE do not, but rather hard code the activations to "relu" for all layers except the final one which is "linear".

Let's change the GraphSAGE class to accept an "activations" argument to specify the activations at all levels. The default behaviour should match the current implementation.

User Story

As a: user of stellargraph library
I want: have similar configuration options for each graph ML class
so that: I can switch between models easily

Done Checklist (Development)

• Assumptions of the user story met
• Produced code for required functionality
• Branch and Pull Request build on CI
• Branch and Pull Request pass unit tests on CI
• Version number reflects new status
• Peer Code Review Performed
• Code well commented
• Documentation in repo
• CHANGELOG.md updated
• Team demo