ENH: enable appending / adding layers to in-memory output file

[brendan-ward]

Follow on to #397

It should be possible to append or add layers to an in-memory output file, provided that file can be re-opened by reading its bytes from memory. Early attempts to do this as one-off operations in #397 were unsuccessful, but might have been running into tangential issues.

Fiona appears to support this using the MemoryFile wrapper object, and using that to hold the in-memory file across multiple write operations.