Fix bug where start/end values are not updated correctly after initialization

[SBlokhuizen]

This fixes a recently introduced bug, where the start_value and end_value would not get updated. self.values_changed = True
is now called at the end of initialization of the tendency to ensure the start_value and end_value are updated accordingly. This fixes an issue where the end_value would not get updated, and the next tendency would take the wrong start value. For example:

waveform:
- {type: sine-wave, base: 8, amplitude: 2, duration: 1.75}
- {type: linear, duration: 2, to: 2}

Would before give:

And now:

Also, this fixes the issue where a warning would be given because the end of previous tendency would not exactly equal the start of the current tendency, due to floating point precision.