Add read! function

[juliasloan25]

Purpose

Adds a new function read! that reads in data to a preallocated space in the DataHandler, as opposed to allocating at each call as the existing read does. This requires the addition of a preallocated_read_data cache to the DataHandler object.

It also requires that read returns a copy of the data in the case of a cache hit to avoid overwriting data in the cache (this was the cause of the bug in the original implementation).

Note that we expect higher allocations when using read due to the added copy, but lower allocations using read! over read. Internally, we now use read! now so users should see reduced allocations.

This feature was initially introduced in #83, but there was a bug in that implementation so it has been reworked here

closes #81