We only want to allow angular input to proj, so it shouldn't say that it
doesn't want angular input. Especially when it does so on receiving
non-angular input.

Since the introduction of other operations types than projections it has
been unclear how proj should treat those. This commit sets record
straight once and for all: proj is only allowed to initialize operations
that take angular input and produces non-angular output, i.e.
projections. This is true to the version of proj before the introduction
of the pipeline and other non-projection operations. Users can still
initiliaze pipelines with proj as long as the resulting concatenated
operation requires angular input and outputs non-angular output.

With projects.h not being available to outside users anymore we need to
define PJ_UNITS, PJ_ELLPS, PJ_PRIME_MERIDIANS and PJ_OPERATIONS
elsewhere.

Related pj_get_*_ref() functions have been removed in favour
of their proj_ namespaced counterparts.

char pointers have been changed to const char pointers.

Resolves #983
Resolved #1147

Make char pointers const

Specify number of decimals to display when transforming coordinates with either proj, cs2cs or cct.

Fixes #920.