OPL-26: added pre/post signal for converter function from calculation rule

[sniedzielski]

Part of ticket: https://openimis.atlassian.net/browse/OPL-26

Here I pushed changes for 'convert' method that has pre/post actions so as to perform db action like read/save invoices not in calculation rule module, but in module from which this entity comes. Thanks to it function 'convert' only focus on data transformations.

This PR should be merged after:

• OPL-26: changes in abstract calculation class due to new calculation rule openimis-be-core_py#85
• OPL-26: changes in calculation module due to new properties from abstract class openimis-be-calculation_py#17
• OPL-26: update rule after changes in abstract calculation rule openimis-be-calcrule_contribution_income_percentage_py#3
• OPL-26: Contribution plan calculation rule: Product modeling openimis-be-calcrule_contribution_legacy_py#1

RELATED PRs:

• OPL-26: Contribution plan calculation rule: Product modeling openimis-be-calcrule_contribution_legacy_py#1

[sniedzielski]

@delcroip I will look into the CI fail with two tests and fix this issue.