The +t_obs parameter was confusing for users since it effectively
overwrote the observation time in input coordinates. To make it more clear
what is the operation is doing, users are now required to directly specify
the time span for which they wish to apply a given deformation. The parameter
+dt has been added for that purpose. The new parameter is mutually
exclusive with +t_epoch. +dt is used when deformation
for a set amount of time is needed and +t_epoch is used (in
conjunction with the observation time of the input coordinate) when
deformation from a specific epoch to the observation time is needed.

Changed the direction of dt-calculation to follow the same convention as
helmert. Changed from dt = t_c - t_obs to dt = t_obs - t_c, which
effectively reverses the direction of the operation. Existing projstrings
using deformation can simply reverse the direction of the operation to
achieve the same results as before this commit.

The source material for the default grids used by
PROJ has been moved to the proj-datumgrid repository. For that
reason it is no longer necessary to include the nad2bin program
in the PROJ repository and source distribution. From now on the
nad2bin application will be kept in the proj-datumgrid repo.

Previously the null grid was generated by running nad2bin on the
null.lla file. Since nad2bin is no longer available null.lla has
been replaced by its binary counterpart null. This file will be
distributed and installed alongside PROJ.

Build scripts and documenation has been adjusted so that nad2bin
is not mentioned anywhere. Additionally all references to nad2nad
has been removed as well. nad2nad has not been part of the PROJ
distribution for quite some time so this has been long overdue.