memory mapped dask arrays

[sophiamaedler]

Building on the memory mapped temp arrays currently implemented in alpha base I created some new functions to use a tempmmap compatible HDF5 in the backend of a memory mapped Dask array (see here : src/sparcstools/base/daskmmap.py). Do you want to include this in alpha base @jalew188 , @mschwoer or do you think its better if I keep it in a separate repo?