Remove classmethods from DeformableModelBuilder

`DeformableModelBuilder` has lots of `@classmethods` on it that don't actually benefit from being class methods (they don't utilize the `cls` variable, nor are they subclassed in any way). This PR simply moves the class methods to be plain functions. All the checking classmethods are moved to `menpo.fitmultilevel.builder.checks`, the rest are left in `menpo.fitmulltilevel.builder`.