Rename `time_explosion`

[andrewfullard]

time_explosion is not clearly defined and leads to confusion. It should be renamed to something more accurate e.g. "time_of_spectrum" (because it is the time at which the spectrum is generated).

There are also other related times such as the time of definition for density and mass fractions.

[wkerzendorf]

I thought about this recently. I think it is not wrong in the sense that it relates to time in relation to the explosion itself (as that is needed to use homology). I feel time_of_spectrum is correct but that does not show what zero point it links to. I'm not sure if time_explosion is the right way forward but I'm not sure time_of_spectrum is either.

[chvogl]

I agree that time_explosion is a bit confusing. However, I would prefer time_since_explosion to time_of_spectrum.

[chvogl]

I agree with @wkerzendorf that time_of_spectrum is somewhat ambiguous regarding the zero point.

[andrewfullard]

The example was definitely intended to provoke discussion!