curveID can now represent multiple columns in userGcMeta

[suz-estella]

Turns out this wasn't too difficult to implement. I added a test that gives a simple example of this.

I did make one change here that I think is better: Before, all 3 input tables userGcMeta, userGcM3, and userGcSPU had to have all columns in sim$curveID. In this update, userGcMeta and userGcM3 both have to have matching 'curveID' columns, but sim$curveID can be any number of columns, as long as they match in userGcMeta and userGcSPU. This makes it so that userGcM3 (a very large table) will never need to have all the same columns as userGcMeta; these tables will remain linked by the single column 'curveID'.

An example of what this could look like (pulled from the new test). userGcSPU no longer needs to have the 'curveID' column:

  SpaDES.project::setupProject(

    modules = "CBM_vol2biomass_SK",
    paths   = list(
      projectPath = spadesTestPaths$projectPath,
      modulePath  = spadesTestPaths$modulePath,
      packagePath = spadesTestPaths$packagePath,
      inputPath   = spadesTestPaths$inputPath,
      cachePath   = spadesTestPaths$cachePath,
      outputPath  = file.path(spadesTestPaths$temp$outputs, projectName)
    ),

    curveID = c("species", "moisture"),

    # Define growth curves and spatial unit combos found in the study area
    userGcSPU = data.frame(
      spatial_unit_id = 28,
      species         = "Balsam Fir",
      moisture        = 50
    ),

    # Growth curve metadata
    userGcMeta = data.frame(
      curveID     = 22,
      species     = "Balsam Fir",
      moisture    = 50
    ),

    # Growth curve columns
    userGcM3 = data.frame(
      curveID     = 22,
      Age         = 0:250,
      MerchVolume = 0:250
    )
  )