`BlockIndex` improvements

[flexatone]

• 1. An unset dtype should return float by default
• 2. Check handling of registering 0 column 2d arrays; might need to understand how to provide shape_reference for cases of rows with no columns, or columns with no rows
• 3. register() should return a Boolean if the block should be held, i.e., it the block has 0 columns we return False
• 4. add properties for rows, columns