Added support to load from memory

[racerxdl]

I added support to load a shapefile from memory. This way if you store the shp/dbf files using go-bindata then you can load it using byte arrays.

    shpFileData, err := ImageData.Asset("ne_50m_admin_0_countries.shp")
    if err != nil {
        panic(err)
    }
    dbfFileData, err := ImageData.Asset("ne_50m_admin_0_countries.dbf")
    if err != nil {
        panic(err)
    }
    shape, err := shp.OpenFromMemory(shpFileData, dbfFileData)
    if err != nil {
        return err, nil
    }
    defer shape.Close()

[houndci-bot]

exported function OpenFromMemory should have comment or be unexported

[codecov-io]

Codecov Report

Merging #36 into master will increase coverage by 0.12%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   60.04%   60.17%   +0.12%     
==========================================
  Files           7        7              
  Lines         801      816      +15     
==========================================
+ Hits          481      491      +10     
- Misses        264      269       +5     
  Partials       56       56

Impacted Files	Coverage Δ
reader.go	61.64% <55.55%> (+0.57%)	⬆️

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[fawick]

Thank you for your PR. I understand that you want to read shapefiles and attributes from data embedded in the executable.

To be honest, the proposed solution feels too "hacky" for me. I am sure that it works, but I'm afraid this could be hard to maintain in the long run.

I'd rather you'd open up an issue to discuss a design. I'd also be interested to get more minds on this.

[fawick]

I'm not happy with adding this member variable to the struct.

Do you need to have random access? If not, have you considered https://godoc.org/github.com/jonas-p/go-shp#SequentialReaderFromExt for opening the byte slices?

[fawick]

IMHO this is a bit of a code smell: Testing whether one of the members is holding data and then branching the logic from there.

[fawick]

I have doubts about this, as well. Effectively, you are adding references to mD twice to s. I understand that you want to preserve the dbf data so that openDbf has something to open later, but the proposed design will overcomplicate things.