Allow to use BioMASS without Graphviz

[himoto]

BioMass requires users to manually install Graphviz for graph visualization but some packages derived from biomass, e.g., pasmpy, do not need it. Currently if we run import biomass without Graphviz, it will raise an ImportError.
I would like to make this optional so that biomass-derived packages can be run without error even if Graphviz is not installed.

Ideas:

• Move pygraphviz and pyvis from requirements.txt to extras_require in setup.py
• Modify graph.py as follows:

def to_graph(self):
    try:
        import pygraphviz as pgv
    except ImportError:
        print("pygraphviz is required to run this function.")
    ...

def dynamic_plot(self, ...):
    try:
        from pyvis.network import Network
    except ImportError:
        print("pyvis is required to run this function.")

@formersbach
I would be grateful if you could give me comments on this. I have a branch working on this and if it's ok, I will send a PR.

[himoto]

This is resolved in #162.