Fix errors where plot region has no data

[abought]

⚠️ Replaces #111 by rendering the error message irrelevant

Purpose

When an association plot is scrolled to a region with no data, cryptic error messages appear. (even after fixing the portal API format) Instead, we would prefer a behavior in which that data layer can render with no data points, and without preventing the rendering of other layers.

Before

After

After fixing the portal API response, it was discovered that the LZLD datasource had a hard-coded assumption of at least one association record.

Summary of changes

Introduces a new concept of "dependent" data sources. This covers sources like LDLZ, which are designed to annotate records fetched from a previous step in the chain. These sources have a strong coupling relationship to another type of data.

Since the LD source only adds meaning in the context of other data: if a prior source (like association) returns no data, dependent sources should skip their own request step entirely.

TODO

• [x] Fix a rendering issue where the "line of significance" data layer is drawn diagonally in the absence of other data . (y-axis scale issues)

Other

The build process now supports creating accurate source maps for locuszoom.app.min.js, to facilitate debugging.

How to test this

This error scenario can be replicated using "offline mode" of the provided index.html example, and using the following as the contents of staticdata/assoc_10_114550452-115067678.json:

{
  "data": {
    "analysis": [],
    "variant": [],
    "chr": [],
    "position": [],
    "refAlleleFreq": [],
    "ref_allele": [],
    "pvalue": [],
    "log_pvalue": [],
    "scoreTestStat": []

  },
  "lastPage": null
}

[abought]

Ben has reported success with these changes. I'll add one more unit test in #123 (since that branch adds sinon support, useful for stubbing data sources).