Add separate message for charging interest without repay

[ethanfrey]

• expose on all markets
• expose on CA to iterate over all markets
• anyone can call this if they want to pay the gas fees
• same logic that we auto-execute when repaying, depositing, etc

[uint]

@ethanfrey Do we need this even if we have #56? Feels like at that point the internal operation of calculating interest would be pretty transparent.

[ethanfrey]

These were ideas from @ueco-jb for testing. If you don't need it to test, then I don't think we need it.

[ueco-jb]

Yup, simulate should suffice in that case.

Closing in favor of #56 .