New interfaces

[kaizu]

• ecell4.extras.units.check_model checks dimensionality of units in a model #83
• Provide ecell4.prelude for easy loading #63
• Enable plotly.js in plotting #42
• Provide Session class for encapsulating a simulation setup #41
• Organize multiple plot functions into ecell4.plotting (ecell4.util.viz is deprecated) #37
• Enable to load parameters for ensemble from a YAML file #28
• Support a new keyword, proceed #19
• First commit to support PySB #16
• Result object for run_simulation #11
• ecell4.plotting.plot_number_observer accepts equations between species in the rule-based manner