Redesign main interface

Close #115 (closed) #111 (closed)

Summary of changes as I go

• Now handles n-parallel properly in addition to any other options
• Deprecates running with multiple samplers: it wasn't possible to specify different sampler-kwargs before anyway so I see the feature as somewhat useless
• Simulations of data with out injections
• Deprecates n-injection for the bilby_pipe input. Instead, use n-simulation (to make sense with creating pure Gaussian noise cases).