disk_index_partition header and cpp

[AtharvaPhadnis]

Create some basic scaffolding for disk_index_partition class and header
No implementations of any methods included yet

Right now we are thinking of the following

Methods

• read_from_disk() : Reads a partition into memory
• write_to_disk() : Flush a partition to disk
• rellocate_disk() : If we need to make any changes to the backing file

Member variables

• file_handler : Either a string that points to the backing file on disk or something more sophisticated like a file handler class for this partition
• is_in_memory : Flag that tracks if this partition is currently resident in memory