Split up `propagate` into lower-level components

[goerz]

Allow to call propagate(propagator) as a low overhead option to propagate over an entire time grid.

Add check_tlist interface routine.

Minor breaking change: renamed showprogress to show_progress

[codecov[bot]]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (b0b96d2) 88.6% compared to head (d56d5f3) 88.2%.

Files	Patch %	Lines
src/interfaces/tlist.jl	70.0%	6 Missing :warning:
src/propagate.jl	68.8%	5 Missing :warning:
src/interfaces/propagator.jl	50.0%	2 Missing :warning:
src/propagator.jl	0.0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #58 +/- ## ======================================== - Coverage 88.6% 88.2% -0.4% ======================================== Files 25 26 +1 Lines 1788 1821 +33 ======================================== + Hits 1584 1605 +21 - Misses 204 216 +12 ```

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