Attributes are saved with a leading underscore

Attributes related to a given source are prefixed with gw<i>_, where <i> is the index of the source. E.g., gw1_df for a galactic binary.

However, global attributes (such as size, dt, or t0) are wrongly prefixed with an extra underscore _, e.g., _size or _dt. We should remove this unnecessary prefixing underscore.