Fix EasyList to accept a list of parameters

[rozyczko]

As noted in #306 (comment) we should be able to instantiate EasyList with a simple list/tuple of parameters.

E.g.

el = EasyList(Parameter('a', value=1.0), Parameter('b', value=2.0))

Currently, EasyList drops those kwargs, as EasyList overrides get_all_variables to only recurse into ModelBase items, not simple sequences and bare descriptors are dropped.

el.get_all_variables()   # []
el.get_fit_parameters()  # []

I am marking this as a bug since any sane person would expect a simple list to be accepted for fitting.

[rozyczko]

This might be as simple as adding this snippet to EasyList.get_all_variables()

for item in self._data:
    if isinstance(item, DescriptorBase):
        all_vars.append(item)
    elif isinstance(item, ModelBase):
        all_vars.extend(item.get_all_variables())