Remove cumulative returns functionalities

[montezdesousa]

Description

This PR removes the cret functionalities. Cumulative returns as implemented are assuming bullet investment at portfolio inception, what's not consistent with the period investment allowed by loading transactions. The total amount invested under periodic investments is constantly changing and cumulative product of 1+return will not capture that.

Example:

Date	Investment	Cumulative investment	Daily return	Cash return	Cumulative cash return
0	1000$	1000$	5%	50$	50$
1	1000$	2000$	2%	40$	90$

Current calculation:
cret = (1+5%)*(1+2%) - 1= 7.1%
cash ret from cret = 7.1% * 2000$ = 142$

Real cumulative return:
Cumulative cash return / cumulative investment = 90/2000 = 4.5%

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