Turn sparse DelayedArray support back on

[lazappi]

Reminder to do this now that https://github.com/grimbough/rhdf5/issues/79 has been fixed.

To do:

• Remove the is_sparse() check
• Update tests
• Remove code coverage guards

[lazappi]

Tried doing this quickly, writing worked ok but it broke reading.

[LTLA]

Will look at this later. I think we need to flip a few flags in h5writeAttributes to properly save it as a UTF-8 string scalar. What was the error?

[lazappi]

I probably should have copied it here but from memory, something along the lines of X is not a valid path.