enables writing of economic models, including two region

[bl-young]

resolves #291

renames buildEIOModel() to buildIOModel()

# Write a two-region model within tests subfolder to excel
modelname <- "GAEEIOv1.0-s-WAT-12"
configpaths <- file.path(paste0('tests/modelspecs/', modelname, '.yml'))
model <- buildIOModel(modelname, configpaths=configpaths)

writeModeltoXLSX(model, ".")

[bl-young]

this has been pulled into develop for the next release (9eeb772) so if we decide this is not urgent we can close this PR

[bl-young]

Will be integrated in v1.6, this commit is already on develop. Closing this PR