Allow fixed loads to have bid prices

[raphaelsaavedra]

In ERCOT, fixed loads submit price offers and are interpreted as "all or nothing" blocks.
We don't have any kind of fixed load cost currently, so we need to add this.
This could be done in two ways: 1) add a new type of bid (other than inc, dec, psl), or 2) allow fixed loads to have bid prices associated with them