Introduce compatibility for cs2cs-style proj-strings into the 4D API.

Parameters such as towgs84, nadgrids and geoidgrids was previously only
handled by pj_transform(). This commit add a compatibility layer in
proj_create() by calling the pj_cs2cs_emulation_setup() function. This
function sets up a handful of predefined transformation objects on the
PJ object that is being created. Each of these transformation objects
are related to the cs2cs-style parameters we are trying to emulate in
the 4D API. That is, if the +towgs84 parameters is used we create
P->helmert with the parameters specified in +towgs84. Similarly for
+axis, +nadgrids and +geoidgrids. When these transformation objects
exists we use them in the prepare and finalize functions in pj_fwd/
pj_inv. If no cs2cs-style parametes are specified we skip those
parts of the prepare and finalizing steps.

Co-authored-by:Thomas Knudsen <thokn@sdfe.dk>
Co-authored-by:Kristian Evers <kristianevers@gmail.com>

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