Relax tolerances in a few unit test, and in laea code.
Seen with gcc 5.3 and also 7.1
Related to the use of the 387 floating-point math, since they
disappear with gcc 7.1 if using non-default -mfpmath=sse -msse

* Make tmerc an alias for etmerc

This switches the algorithm used in tmerc to the Poder/Engsager
tmerc algorithm. The original tmerc algorithm of Evenden/Snyder
origin can still be accessed by adding the +approx flag when
initializing a tmerc projection. The +approx flag can also
be used when initializing UTM projections, in which case the
Evenden/Snyder algorithm is used as well.

If a tmerc projection is instantiated on a spherical earth
the Evenden/Snyder algorithm is used as well since the
Poder/Engsager algorithm is only defined on the ellipsoid.

+proj=etmerc can still be instantiated for backwards compatibility
reasons.

Co-authored-by: Kristian Evers <kristianevers@gmail.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>

This communicates better what the unit actually is. It also opens the
door for potentially adding PJ_IO_UNITS_DEGREES in the future in order
to coordinates in degrees correctly as well.