BUG: null value in dataset

[giraud10]

When you pass to sigo a dataset where there are missing values sigo stops.

{"species":"Adelie","island":1,"bill_length_mm":39.1,"bill_depth_mm":18.7,"flipper_length_mm":181.0,"body_mass_g":3750.0,"sex":0,"year":1167609600}
{"species":"Adelie","island":1,"bill_length_mm":39.5,"bill_depth_mm":17.4,"flipper_length_mm":186.0,"body_mass_g":3800.0,"sex":1,"year":1167609600}
{"species":"Adelie","island":1,"bill_length_mm":40.3,"bill_depth_mm":18.0,"flipper_length_mm":195.0,"body_mass_g":3250.0,"sex":1,"year":1167609600}
{"species":"Adelie","island":1,"bill_length_mm":null,"bill_depth_mm":null,"flipper_length_mm":null,"body_mass_g":null,"sex":null,"year":1167609600}

panic: interface conversion: interface {} is nil, not float32