Improved weight treatment in plotting scripts

This is addressing a number of things:

• if a file has no clearly defined default weight, it is now possible to specify the default weight on the command line, like so:

rivet-mkhtml --errs file.yoda:"DefaultWeight=variationXYZ"

• if plotting two yoda files corresponding to different predictions with different weights names, it is now possible to specify different subsets as well, like so:

rivet-mkhtml --errs file1.yoda:"Variations=Var1,Var2,Var3" file2.yoda:"Variations=MUR.*MUF.*PDF123456"

(Note that passing Variations=none will typically match no weight name and will effectively switch them off for that particular file. It is also still possible to pass the global flag --no-weights which will drop the variations for all files.)

• Similarly, it is possible to specify different envelope prescriptions:

rivet-mkhtml --errs file1.yoda:"BandComponentEnv=Var1,Var2,Var3" file2.yoda:"BandComponentEnv=MUR.*MUF.*PDF123456"

• It is now possible to specify the same file twice on the command line, provided a different "Name" is passed as a file argument. This allows the user to compare two variations in the same file, like so:

rivet-mkhtml --errs file.yoda:"Name=nominal:Variations=none" file.yoda:"Name=variations:DefaultWeight=Var1:Variations=none"