add methods to set up custom workflows/optimisations

[fzahle]

based on inputs written in the yaml input file we can allow users to set up optimisations. pierre has set this up in topfarm

https://github.com/DTUWindEnergy/TOPFARM/blob/master/examples/notebook/tutorial2_advanced.ipynb
https://github.com/DTUWindEnergy/TOPFARM/blob/master/src/topfarm/topfarm.py#L106

where he passes a dictionary to the assembly that then adds objective, cons and params:
components{ 'driver': 'PyOpt':dict(method='ALHSO',
hms=10,
hmcr=0.8,
par=0.65,
etc...),

objectives =  {'driver': '0.5 * foundation.foundation_length + 0.5*elnet.elnet_length'}

constraints = {'driver': ['wt_dist.min_dist>0.8', 
                      'elnet.elnet_length<1.1', 
                      'dist_from_borders'
                      ]}

design_variables = {'driver': 'distribute'}

use the Assembly.replace method on the existing assembly and then instrument it with objectives cons and params pretty much like Pierre does it.