web3.utils.fromWei
将给定的以wei为单位的值转换为其他单位的数值。
注意，wei是最小的以太单位，应当总是使用wei进行计算，仅在需要显示 时进行转换。
调用：
web3.utils.fromWei(number [, unit])
Converts any wei value into a ether value.
参数：
number - String|Number|BN: wei为单位的数值
unit - String，可选，默认值为ether，可选的单位有：
- noether: ‘0’
- wei: ‘1’
- kwei: ‘1000’
- Kwei: ‘1000’
- babbage: ‘1000’
- femtoether: ‘1000’
- mwei: ‘1000000’
- Mwei: ‘1000000’
- lovelace: ‘1000000’
- picoether: ‘1000000’
- gwei: ‘1000000000’
- Gwei: ‘1000000000’
- shannon: ‘1000000000’
- nanoether: ‘1000000000’
- nano: ‘1000000000’
- szabo: ‘1000000000000’
- microether: ‘1000000000000’
- micro: ‘1000000000000’
- finney: ‘1000000000000000’
- milliether: ‘1000000000000000’
- milli: ‘1000000000000000’
- ether: ‘1000000000000000000’
- kether: ‘1000000000000000000000’
- grand: ‘1000000000000000000000’
- mether: ‘1000000000000000000000000’
- gether: ‘1000000000000000000000000000’
- tether: ‘1000000000000000000000000000000’
返回值：
String|BN: 如果输入参数是数值或字符串，则返回字符串，否则返回BN.js对象
示例代码：
web3.utils.fromWei('1', 'ether');
> "0.000000000000000001"
web3.utils.fromWei('1', 'finney');
> "0.000000000000001"
web3.utils.fromWei('1', 'szabo');
> "0.000000000001"
web3.utils.fromWei('1', 'shannon');
> "0.000000001"