Move special imports to `extras_require`

[j6k4m8]

You should be able to use DotMotif without installing neo4j- and docker-flavored dependencies.

Proposal:

# Only install core dotmotif for networkx and grandiso:
pip install dotmotif

# Install cypher extensions for neuPrint, and docker 
# extensions for creating a local neo4j database:
pip install 'dotmotif[neo4j]'