Docstring for ETdetector.response_function()

	modified:   modules/ETdetector.py
Set ET's detector locations and detector tensors

These are given in the Earth-Centred Earth-fixed frame.

Numbers obtained from 'Mdc_ET_v2.c' from the ET-MDC code
(https://svn.oca.eu/codes/MDC_Generation/MDC_Generation/trunk/Mdc_ET_v2.c)

	modified:   modules/ETdetector.py
Test written for ETdetector.overlap_exp()

	new file:   tests/ETdetector/overlap_exp.py
Test passed for ETdetector.overlap_exp()

Routine computes the exponential factor in overlap-reduction function:

\exp{ i 2\pi f \hat{\Omega} \cdot (p_I - p_J) }

	modified:   modules/ETdetector.py