dustfrac and dust-to-gas not correct when ndusttypes > 7

[markahutch]

When calculating the additional quantities "dustfrac" or "dust-to-gas" with more than 7 dust phases, the string length (80) is not long enough to parse the function. Unfortunately this does not throw an error, which makes it appear as though there is an error in the data file when there is not.

A quick fix would be to use a longer string length, but perhaps a better solution would be to add the ability to parse the string "sum(dustfrac)" as "dustfrac1+dustfrac2+..."

This would also help with working with deltav, e.g. when calculating differential velocities between dust phases.