pgSphere used its own setting (set_sphere_output_precision function)
to limit the output precision of double values, that could not be
greater than 15 significant digits (DBL_DIG). It introduced some
problems with dump/restore. PostgreSQL uses its own precision setting:
extra_float_digits. The PostgreSQL setting allows to use more significant
digits.

This patch changes the default pgSphere output behaviour to utilize
PostgreSQL extra_float_digits. Now, extra_float_digits is used by default,
until set_sphere_output_precision is called.

The old behaviour is kept for compatibility purposes. Once,
set_sphere_output_precision is called, pgSphere starts to use the old
behaviour (read, please, issue postgrespro#118 discussion).

The patch introduces a new function - reset_sphere_output_precision.
It is used to reset to the PostgreSQL behaviour after using
set_sphere_output_precision.

expected/output_precision.out   - PG 10-11
expected/output_precision_1.out - PG 12+