Allow nested pipelines. (#629)

Allow nested pipelines when wrapped in +init's.

The previous behaviour was to quit pipeline initialization when
encountering a nested pipeline definition. With this commit that
behaviour is changed so that it is possible to nest pipelines as
long as they are defined elsewhere in a init-file.
This is useful in init-files where steps in complicated transformations
can be grouped in "sub-pipelines". These "sub-pipelines" can then be
used as individual steps in a larger and more complicated pipeline.

Nested pipelines are governed by the following rules:

1. You can't have more than one literal +proj=pipeline in a proj-string
2. Pipelines can be nested if they are wrapped up in a +init
3. More than one +init is disallowed in non-pipeline proj-strings
4. +inits are expanded as late as possible, that is they will only be
    expanded in single operations (that can be a part of a pipeline)

1 files changed, 1 insertions, 0 deletions

diff --git a/src/projects.h b/src/projects.h
index 1fe595bc..6ea0c914 100644
--- a/src/projects.h
+++ b/src/projects.h
@@ -532,6 +532,7 @@ struct FACTORS {
 #define PJD_ERR_MISSING_ARGS            -54
 #define PJD_ERR_LAT_0_IS_ZERO           -55
 #define PJD_ERR_ELLIPSOIDAL_UNSUPPORTED -56
+#define PJD_ERR_TOO_MANY_INITS          -57
 
 struct projFileAPI_t;