Add resolutionMultiplier config slot to BigWigAdapter

[cmdcolin]

Allows a default multiplier to adjust the initially shown resolution in bigwig files

So e.g. setting it to 10 makes it quite low resolution on a whole genome coverage bigwig file

http://jbrowse.org/code/jb2/resolution_multiplier/?config=test_data%2Fconfig_demo.json&session=share-8ItYciPZqJ&password=LsY0O

the example config

    {
      "type": "QuantitativeTrack",
      "trackId": "ngmlr_cov",
      "name": "SKBR3 pacbio coverage (NGMLR)",
      "assemblyNames": ["hg19"],
      "adapter": {
        "type": "BigWigAdapter",
        "resolutionMultiplier": 10,
        "bigWigLocation": {
          "uri": "https://s3.amazonaws.com/jbrowse.org/genomes/hg19/reads_lr_skbr3.fa_ngmlr-0.2.3_mapped.bam.regions.bw"
        }
      }
    },

[cmdcolin]

fixes #4618

[cmdcolin]

there are some further subtle points of discussion at #4618 but this PR fixes the main issue and @carolyntcaron confirmed it worked for their use case