Optimize storing fill_value to Array

[jakirkham]

Would be nice if when storing a scalar or an array that contains only the fill_value of the Array, the chunk is merely deleted (if it exists) instead of writing out a chunk that contains only the fill_value for all elements. The Array should behave identically in either case. The only benefit is that storage is cutdown automatically in this case. A situation where this may come up is when copying one Array to another.

[jakirkham]

A rough attempt at solving this is in PR ( #367 ).