Remove node_features, edge_features, graph_features methods

[calebjubal]

Summary

This PR removes the node_features, edge_features, and graph_features helper functions and updates all usages to direct property access.

Motivation

These convenience functions were problematic because:

1. They only worked when there was a single feature array in ndata/edata/gdata

2. They threw an error when multiple feature arrays existed, which was confusing

3. Direct property access (g.ndata.x, g.edata.e, g.gdata.u) is more explicit and flexible

Changes

Removed:

• node_features(g::GNNGraph) function definition
• edge_features(g::GNNGraph) function definition
• graph_features(g::GNNGraph) function definition
• Exports from GNNGraphs module

Updated to use direct property access:

• basic.jl - GNNLayer and GNNChain forward passes
• conv.jl - GATConv, GATv2Conv, NNConv, CGConv, MEGNetConv, GMMConv, TransformerConv
• pool.jl - GlobalPool, GlobalAttentionPool, Set2Set
• pool.jl - GlobalPool, GlobalAttentionPool
  Test files updated accordingly

Migration

Users should replace:

# Before
x = node_features(g)
e = edge_features(g)
u = graph_features(g)

# After
x = g.ndata.x  # or whatever key was used
e = g.edata.e
u = g.gdata.u