Updated exponential fitting to make it robust.

Updated exponential fitting to make it robust.

The exponential fitting code was not robust under different
data. This is improved by ensuring that e.g. time constants
are always positive. For multi-exponential fits the time constants
are guaranteed to be in increasing order. Rewrote test code.

Added test for error estimation that reproduces 5.0 behavior.

Added new manual section "Curve fitting in GROMACS" (8.6).

General cleanups of the code were done.

Change-Id: Ib72fccf7f85742afeeb3fc0fd6fbd44c1c47795a