New example using the FLORIS wake model

Added possibility to specify units

Created examples/docs/example_6_floris.py where floris and pywake are compared

added Plant Energy and Floris to CI-image

Introduced a plug-in type architecture

extended possible design variable input types:

• [value]
• [(value,'unit')]
• [(value, lower, upper)]
• [(value, lower, upper, unit)]