non final capture + return amount + more documentation

[devkral]

Add final parameter for final captures. Defaults to true for old logic.
Changing it allows multiple captures.
First only paypal is supported but I have some providers in pipeline which use it too.
There is no check if captured_amount is bigger than total because some providers allow overcapture.

Secondly the amount or None is returned in refund and capture (BasePayment). This streamlines the return behaviour of Providers.

At last I add some documentation for the methods.

[codecov-io]

Codecov Report

Merging #148 into master will decrease coverage by 0.05%.
The diff coverage is 89.65%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
- Coverage   79.13%   79.08%   -0.06%     
==========================================
  Files          27       27              
  Lines        1649     1659      +10     
  Branches      190      195       +5     
==========================================
+ Hits         1305     1312       +7     
- Misses        245      246       +1     
- Partials       99      101       +2

Impacted Files	Coverage Δ
payments/cybersource/__init__.py	66.45% <100%> (ø)	⬆️
payments/dummy/__init__.py	86.79% <100%> (ø)	⬆️
payments/core.py	87.69% <100%> (ø)	⬆️
payments/paypal/__init__.py	84.58% <100%> (ø)	⬆️
payments/stripe/__init__.py	60% <100%> (ø)	⬆️
payments/models.py	74.41% <87.5%> (-0.38%)	⬇️

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