add support for data installations

[smoors]

motivation

• leverage EB to install data in a standardized way with proper versioning and checksumming
• support adding datasets as dependency for software
• easily swap dataset versions with ml swap

changes

• add cmd line option --installpath-data similar to --installpath-software
• add cmd line option --subdir-data (default = data) similar to --subdir-software
• add cmd line option --sourcepath-data similar to --sourcepath
• add Easyconfig parameter data_sources similar to sources

design

• the main reason for a separate subdir_data is reusability: in contrast to software it does not have to be rebuilt/reinstalled when for example upgrading the OS or building for a new architecture
• the reason for a separate sourcepath_data is that datasets can be very large, so you may want to store them in a different file system or location.