Remove toWEI and fromWEI in favour of ethers.utils

[bh2smith]

Documentation here: https://docs.ethers.io/v4/api-utils.html#ether-strings-and-wei

THis would mean we no longer need to unit test the functions as they are already tested elsewhere.

[bh2smith]

BTW @schmanu - this change is going to be incredibly painful since we have to change the BigNumber type from bignumberjs to ethers. I have already started porting this and will link a draft PR here. Some things we will have to deal with:

1. bignumber.js works on floating points as well, while the ethers one seems to only support integers.
   This means that BigNumber.from("1.1") is no longer a thing.
2. We also have in some places (for NFTs) assertions that tokenID is not NaN and isInteger both of these functions don't exist on ethers.BigNumber (however once we get the floating point issue sorted out these checks may no longer be valid).

Here is the draft PR