A reinvention of the JSON-to-MARC code for MUSICat.

The purpose of this is to put MUSICat's MARC export capability on a sound footing. (Because, Dear Reader, I myself wrote the hasty and entirely crap implementation presently in place.)

Goals of this very short project will include:

• A sane and sensible abstract representation of the MARC record data.
• Externalization of rendered data forms.
• Externalization of per-library preferences as to inclusion/exclustion of data items.
• Externalization of per-library preferences as to minor formatting tweaks.
• A useful set of commandline utilities for working with MARC.
• Understandable code for those who will come after.
• MVP in just a few workdays.

So, here's what this is: a complete component that isolates MARC-v-JSON data and formatting for MUSICat standard form. Will implement as a set of Python commandline tools, then as a Javascript code module that can be pulled into musicat-api.

The MARC formats will be:

• ISO-2709 binary (original MARC byte-oriented format)
• MARC text as developed by Rabble, CCR, and SPL for editor-based ingestion.

"JSON" format does not refer to the "MARCjson" proposed form. It means

There will be roundtripping (as time permits) where there is no data loss.

Many cataloging systems have practices that are surely not general usage, but that Rabble is obligated to continue to support.

A real-world example:

```
else if (fieldcode == "650") {
    if (line.substring(10) == "Rap (Music)") {
        termfullstop = false;  // do not terminate the entry with a period.
    }
}
```

This needs to be easily understood, found, modified, and implemented: preferably in config declaration rather than in code.