Add reasonable bounds for FM90

[karllark]

Work on MW and SMC has resulted in reasonable default bounds for the FM90 shape model. Should be added to the definition of this model. Can always be overridden.

Code snippet with values:

  fm90_init.C1.bounds = (-2.0, 3.0)
   fm90_init.C2.bounds = (-0.1, 1.0)
   fm90_init.C3.bounds = (0.0, 2.5)
   fm90_init.C4.bounds = (0.0, 1.0)
   fm90_init.xo.bounds = (4.5, 4.9)
   fm90_init.gamma.bounds = (0.6, 1.5)