Add parsing support for gift transactions (Schwab Equity Awards JSON).

[m01]

This adds a new action type, as TRANSFER seems to deal with money transfers rather than share transfers, and SALEs also involve money whereas gifted shares do not.

The calculation code will raise a NotImplementedError if it encounters one of these transactions.

This may help #510. This contribution has been developed in my spare time.

[vmartinv]

If adding proper handling for gifts is out of the table, I think a better fix would be to raise an unknown transaction error in the parser if clarity is your concern. I see no need to add parsing for an unsupported transaction.
If a new contributor decides to implement gifts for a different parser they will have to worry about them working with Schwab which adds more work.

[m01]

If adding proper handling for gifts is out of the table,

It's not off the table - I plan to look into it but I don't really want to promise that I'll end up implementing it either at this stage.

If a new contributor decides to implement gifts for a different parser they will have to worry about them working with Schwab which adds more work.

I thought this would help progress support for gifts by one step, but also I don't mind if you want to hold off merging this until there's code using it for more than just error handling.

[m01]

So #549 adds support for a TRANSFER_TO_SPOUSE action type.

This PR and #549 could be linked up (and based on local testing it seems to work). The question is, how should gift transactions be identified as being to a spouse (as other types of gift may be taxed differently)? The data export doesn't contain enough data to determine that.

Possible approaches I've thought about:

• Assume all gift transactions are transfers to a spouse, possibly only if some flag is supplied.
• Depend on some extra data somewhere (maybe in a simple CSV file) that identifies transfer to spouse transactions.

I'm not sure what the best approach is for this project.