Support agent modified orders

[masanorihirano]

Because of python, we cannot protect orders submitted once.
Therefore, agents can modify the submitted orders if they store submitted order instances.

pams simulation architecture limit the chance agent can modify the orders because usually agents can act something only in submit_orders methods.
However, we should consider the processing of modified orders.

Due to the original plhamJ implementation, currently there is no way to modify the submitted orders and only thing agents can do is cancel orders and submit new orders.

Note: todo is written here:

pams/pams/market.py

Line 432 in 8090db6

# ToDo: Agent modification will be handled in simulator

[masanorihirano]

this is not urgent and careful consideration is required.
Moreover, we should write the document about it