Errors in Codec of MPTAmount

[ckeshava]

As detailed in this comment, there are two issues with the codec of MPTAmount.

1. Non-str inputs are not handled correctly in the serialization of MPTAmount.value .
2. In the str type inputs, scientific notation is not handled.
3. It remains to be seen if IOU and XRP currencies correctly handle the above two issues. Appropriate unit tests need to written to that effect.

This PR reduces the API interface of the MPTAmount class to str type inputs only. It provides helpful type-hints and error-message to the users until such time as points-1 and 2 are addressed.