Returns true if value is a BigNumber instance, otherwise returns false.

If BigNumber.DEBUG is true, throws if a BigNumber instance is not well-formed.

x = 42
y = new BigNumber(x)

BigNumber.isBigNumber(x)             // false
y instanceof BigNumber               // true
BigNumber.isBigNumber(y)             // true

BN = BigNumber.clone();
z = new BN(x)
z instanceof BigNumber               // false
BigNumber.isBigNumber(z)             // true