Pad compound datatypes to preserve field offsets

[mattjala]

In order to fix field offsets in compounds not being preserved (HDFGroup/hsds#273), the VOL should create padding fields to send to the server. The server will treat these padding fields as regular fields on a compound type, preserving the position of the original fields. When the VOL later reads the datatype, the padding fields should be replaced with byte offsets in a reconstructed datatype.