Feat/utility func change

Proposed changes

This PR introduces a change to the utility function and therefore also to the competitive equilibrium calculation.

Previously, the utility function component for good j had to be capped if the quantity of the good j was 0. Now this is no longer necessary as we shift the quantities such that the value in the natural logarithm is always positive. The default shift is QUANTITY_SHIFT = 1. This change does not affect the trading behaviour of the agents.

Fixes

This addresses the feature request #106 .

Types of changes

What types of changes does your code introduce to agents-tac? Put an x in the boxes that apply

[ ] Bugfix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

[x] I have read the CONTRIBUTING doc
[x] I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
[x] Lint and unit tests pass locally with my changes
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have added necessary documentation (if appropriate)
[x] Any dependent changes have been merged and published in downstream modules

fetchai / agents-tac