Gem Join and Exit to not match documentation

[brianmcmichael]

Documentation for gem join and gem exit indicate that user should pass a wad value, however the actual implementation converts these values into their input * (10**18).

Either the documentation should be updated or the code should. There will be ancillary effects that need to be accounted for, such as in assumptions being made in mcd test

[brianmcmichael]

Additionally, the implementation for these needs to be updated to account for the actual decimals for each gem, rather than assuming 10**18

[brianmcmichael]

Fixed.