Make Measured::{Weight,Length} more comprehensive

[thegedge]

There's a lot of units missing from Measured::Weight (e.g., tonne) and Measured::Length (e.g., kilometer), so let's make them more comprehensive.

[kirinrastogi]

Hi @thegedge I wanted to help with this in the Measured::Weight units. What units should be added? Newtons, Tonnes, Slugs, Short Tons, Milligrams?

Also, for conversions, are we assuming they will all be done close to the surface of the earth?

I was going to style this like #85 if you needed help on this.

[thegedge]

Hey, @kirinrastogi. Thanks for the interest in helping out!

I didn't have any specifics as to which units to include, but I think "as many commonly used lengths and weights" would be reasonable guideline, which we can hopefully grab from Wikipedia or some other site that does a good job of enumerating these units. We can always add more later :)

#85 does make a good template for adding in new units. We'll merge that PR, so no worries about kilometres and miles.

Before working on this I think it would be good to do #88 first, which will make it far easier to build unit systems with units that allow SI prefixes.

[kirinrastogi]

@thegedge thanks for the quick response! I have already started on weights, so I will continue with non si weights (Tonne, Newton etc) and save milligrams and the like for after #88 is completed.

Is that ok?