Allow fixed loads to have bid prices

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

Start by reviewing the existing bid types and fixed-load definitions in the Julia market-clearing model. Compare adding a new bid type with allowing fixed loads to carry bid prices; done should mean fixed-load price offers are represented and interpreted as all-or-nothing blocks.

Written by the indexing model from the issue text.