A Function to update the unit conversion object

[kaustubhmote]

A function update_uc is added to fileiobase to update an existing unit_conversion object. This resolves #219. It will work something like this in practice:

dic, data  = ng.bruker.read(filepath)
udic = ng.bruker.guess_udic(dic, data)
uc = ng.fileiobase.uc_from_udic(udic)

new_car = uc._car + 10 
uc_new = ng.fileiobase.update_uc(uc, car=new_car) 

# sw, obs, cplx, size can also be updated in a similar manner 

I thought about implementing this in the object oriented interface (uc.update(car=new_car), but maybe this functional way is better for the usual reason of avoiding side effects.