[RTM after next push] fix: linear continuous multiphase continuity

[Ipuch]

All Submissions:

• [x] Have you followed the guidelines in our Contributing document [docs/contribution.md]?
• [x] Have you checked to ensure there aren't other open [Pull Requests] for the same update/change?
• [x] Have you opened/linked the issue related to your pull request? #844
• [x] Have you used the tag [WIP] for on-going changes, and removed it when the pull request was ready?
• [x] When ready to merge, have you sent a comment pinging @pariterre in it?

New Feature Submissions:

1. [x] Does your submission pass the tests (if not please explain why this is intended)?
2. [x] Did you write a proper documentation (docstrings and ReadMe)
3. [x] Have you linted your code locally prior to submission (using the command: black . -l120 --exclude "external/*")?

Changes to Core Features:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new examples for your core changes, as applicable?
• [x] Have you written new tests for your core changes, as applicable?

---

This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.34146% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.30%. Comparing base (944c2fe) to head (01d3962). Report is 34 commits behind head on master.

:exclamation: Current head 01d3962 differs from pull request most recent head 9952c5b. Consider uploading reports for the commit 9952c5b to get more accurate results

Files	Patch %	Lines
bioptim/limits/phase_transition.py	90.00%	1 Missing :warning:
bioptim/limits/phase_transtion_builder.py	97.14%	1 Missing :warning:
bioptim/optimization/optimal_control_program.py	96.77%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #857 +/- ## ========================================== + Coverage 78.25% 78.30% +0.04% ========================================== Files 140 141 +1 Lines 16230 16278 +48 ========================================== + Hits 12701 12746 +45 - Misses 3529 3532 +3 ``` | [Flag](https://app.codecov.io/gh/pyomeca/bioptim/pull/857/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pyomeca/bioptim/pull/857/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca) | `78.30% <96.34%> (+0.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[Ipuch]

• see #858