Synchrotron losses time evolution

[grzegorzbor]

The ParticleDistribution.evaluate_time() function added.
It's a basic method that accepts the energy losses function as the parameter, and evaluates it for the requested time period (with optional subperiods).
For now just the first, basic version. It can be later extended to be smarter - for example automatically use shorter time ranges for the energies where higher losses occur.
For now only synchrotron losses implemented, other will be added later.
We can later add the higher level function that would take the flags which radiative processes should be used, instead of accepting generic energy loss function as the parameter.

[jsitarek]

thanks for the PR.
I put some comments

[jsitarek]

thanks for the changes, everything looks good to me now.
let's see what @cosimoNigro thinks

[grzegorzbor]

Regarding the 2 failing tests for agnpy-fit, they are not related to this PR.
It seems there is some issue with the new version of gammapy 1.2, released on February 29.
I ran these tests locally and they work fine with gammapy 1.1, but fail with gammapy 1.2.
So i expect the master branch to also fail after 29th of Feb.

[jsitarek]

then I think the simplest solution for the moment would be to fix in the environment file gammapy version to ~1.1.
Actually @cosimoNigro why gammapy is installed via pip and not directly?