Only read h5parm timestep values that are necessary

Functionality to calculate the axis is refactored into a new function, and a unit test is added for this. The new function behaves slightly different in a situation where interpolation is required, a single time is requested and the time value matches exactly with the axis; I think the new behaviour is better but I would like to confirm with Tammo Jan.

I changed one variable from its name "time_and_frequencies" to "tf". I know this may be considered not according to Google style, but it was used tens of times in a small block of code and the long name makes the code seriously unreadable — there is clearly a limit to plainly applying Google style to everything.