Importing featuremapper before topo sets wrong Time type

[philippjfr]

When importing featuremapper the Dimensions are defined along with the Dimension types:

# Time features
Time     = Dimension("Time", type=param.Dynamic.time_fn.time_type)
Duration = Time("Duration")

However before topographica has been import the param.Dynamic.time_fn.time_type is defined as int, which means that all non-integer Time and Duration values will be cast to int.