Issue 55: Add fitdistdoublecens wrapper

[seabbs]

Description

This PR closes #55 by improving support for fitdistrplus vis a new wrapper function fitdistdoublecens. Unfortunately it does so by assigning temporary functions to the global environment as this was the only solution found to work with fitdistrplus. This means that it may not be CRAN compliant opening a new issue for this).

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

LGTM (self-review)