Mag indices are 1-based
Create synth module and Synth class

The Synth module coordinates all inputs for the StarFISH
synth program, including crowding files, extinction files
and setup the CMD planes to compute.
Create Lockfile class

Lockfiles tie generate isochrones together to reduce dimensionality
in star formation history fits. These lockfiles are needed by synth.

Lockfile depends in having an `isofile` to list the installed
isochrones.

Currently we have a Lockfile.lock_box method to group together
isochrones in a box of age and metallicity space. Unlocked
isochrones are put into singleton groups by default.
Create ExtinctionDistribution

Currently ExtinctionDistribution is fairly rudimentary:
you can either configure uniform extinction by adding a single
extinction value, or using an arbitrary extiction *distribution*
by adding a list of extinction values sampled from that distribution.

In the future it may be good to include some geometric dust models
to help set this up.
Add isofile_path property to LibraryBuilder