New generic accessors HLC3, HL2 and OHLC4

[priikone]

Add HLC3 ((h + l + c) / 3), HL2 ((h + l) / 2) and OHLC4
((o + h + l + c) / 4) as generic accessors that can be used with different
indicators. Most indicators access the data with p.accessor(d) and with
the new accessors they can now get the data in various different forms
(and not only the close price what OHLC accessor provides).

Change the VWAP to use p.accessor(d) and HLC3 accessor by default.

[andredumas]

Thanks again for the contribution.

Let me have a think about this one. The purpose of accessors is to allow users to redefine how elements are read from their data. By redefining the default method to a calculation it changes the purpose somewhat. I feel these are very specific to indicators or calculations rather than data accessors.