Use expression to generate Index bands

[raphaelrpl]

We should support a way to generate custom index band using expression to represent those values. For example, the code below is a example how to generate Normalized Difference Vegetation Index ((nir - red) / (nir + red)). To create a new data cube, which must have NDVI, the curl request should looks like:

{
    "datacube": "LC8",
    "grs": "BDC_GRID",
    "resolution": 30,
    "temporal_schema": "A16day",
    "composite_function": "STK",
    "bands_quicklook": [
        "band6",
        "band5",
        "band4"
    ],
    "bands": [
        { "name": "band1", "common_name": "coastal", "data_type": "int16" },
        { "name": "band2", "common_name": "blue", "data_type": "int16" },
        { "name": "band3", "common_name": "green", "data_type": "int16" },
        { "name": "band4", "common_name": "red", "data_type": "int16" },
        { "name": "band5", "common_name": "nir", "data_type": "int16" },
        { "name": "band6", "common_name": "swir1", "data_type": "int16" },
        { "name": "band7", "common_name": "swir2", "data_type": "int16" },
        { "name": "Fmask4", "common_name": "quality", "data_type": "uint8" }
    ],
    "indexes": [
        { "name": "NDVI", "common_name": "ndvi", "data_type": "int16", "value": "(nir - red) / (nir + red)" },
        { "name": "EVI", "common_name": "evi", "data_type": "int16", "value": "2.5 * ((band5 – band4) / (band5 + 6 * band4 – 7.5 * band2 + 1))" }
    ],
    "quality_band": "Fmask4",
    "description": "Data cube Landsat-8 with period 16 day in year.",
    "license": "MIT"
}

(The user can set any formula as wish, according to bands name defined)

The top-level member indexes act as post-processing to generate data cube indexes. To do that, we must store the value in database and it depends from bdc-db.

Right now, in the commit e137790 we already added a indexes key, but it is hardcoded way and does not support dynamic expression like that.