Add TEOB-HMs and massively parallel cpnest

This merge request implements two separate improvements:

1. A massively parallel version of cpnest, compatible with this branch;

2. Higher modes of the TEOB model.

Additionally, there are small generalisations of the template start-time handling and of the maximum entropy psd usage.

Before merging, the following tests/modifications are required:

• Add a flag to switch on/off the massively parallel cpnest implementation (default: massively parallel off), to save compatibility with cpnest master. This allows both to preserve backwards compatibility and avoid disrupting the gitlab CI. When this branch will become the master cpnest version, this option will need to be modified accordingly;

• Add a test config and a standard simulation config of the TEOB model (updating this one to showcase HMs usage), and a quickstart config, similar to the ones found here.

• Running a standard injection and a single real event comparing cpnest parallel with the current master version, using any model (since the default will still be not parallel, there is no need for massive numbers of tests).

• Describe why the likelihood.pxd file was removed, in contrast with the other cython files.

If future LVK usage is foreseen, pp-plots should be produced, similarly to what was done here.