Refactor LibraryBuilder parameters

All parameters for lib.dat are now set in `LibraryBuilder.__init__`.
This allows us to create a valid `LibraryBuilder` instance without
having to run `mklib` itseful. This is useful because `LibraryBuilder`
is used by other classes as storage for paths and parameters.
Paths to the isofile and lib.dat are also set at `__init__`.
Synth.add_cmd can accept label for CMD axes

These labels are meant to aid in plotting synthetic CMDs.
Synth plots Hess diagrams of synthetic CMDs

The `Synth.plot_all_hess` method is a wrapper for automating the
plotting of all available synthetic CMDs. The actually plotting
is done by `Synth._plot_hess`.

If synthetic CMD file is not available, and error is logged
and the `plot_all_hess` wrapper continues. I don't know why
some entires in the `isofile` are being skipped.