Shared sdram

[Christian-B]

Designed to be used with #581

Removals

1. Subtraction between sdram object
   -Unused and make even less sense with SharedSdram
2. vertet json mehtods

• Unused. Can be replaced when the use case defines what is needed.

Changes.

1. eq defintion moved to each sdram
2. MultiRegionSDRAM now holds total as another sdram object

• Many methods delegated to that sdram object

4. MultiRegionSDRAM add_cost now uses nest method (as did the same thing)
5. add methods changed to handle the extra type

SharedSdram
-Takes a dict of keys to sdram Objects

• Optionally also a per_core sdram (mainly when summing)