When fitting extra frequencies, don't forget the uncertainties.

If you manipulate an ArrayQuantity.value_si array without also
changing the uncertainty array then you can end up with arrays
of different sizes, which causes a problem when you pickle them
and unpickle them (eg. write and load a restart file).

Hopefully addresses #114
Check uncertainties size when setting value on an ArrayQuantity

If you set the value on an ArrayQuantity it should ensure you
have the right number of uncertainties. Currently the only way
to change both would be to make a new ArrayQuantity object,
or to modify the uncertainty array first then set the value.

You can currently bypass this check by changing the value_si array.

There maybe ought to be more error checking in ArrayQuantity in general.
(Unless that slows it down too much - perhaps only in a debug mode).