Add turku_spatial
Add roxygen docs for get_turku_adminboundaries

Based on get_helsinki_aluejakokartat
Add cache dirs to gitignore
Add new utility function for downloading data
download_data operational
Add major-districts to turku_spatial
Read the actual spatial data in turku_spatial

Includes data sources 'elections' and 'major-districts'
Update docs and NAMESPACE
Migrate helsinki_spatial to the same structure as turku_spatial

Data downloading and extraction outsourced to download_data()
Update docs