Fix 440 EICRecon compilation errors if used as cmake subproject

[DraTeots]

In Cmake not there is EICRECON_SOURCE_DIR points to repository root which is used in jana_plugin.cmake and other places that need this particular place to be referenced

Briefly, what does this PR introduce?

What kind of change does this PR introduce?

• Bug fix (fixes issue Compilation error if EICrecon is used as cmake subproject #440)

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

no

Does this PR change default behavior?

no

[wdconinc]

I'm surprised there is a need for a new EICRECON_SOURCE_DIR. CMake provides so many *_DIR variables already that I doubt this is necessary...

[veprbl]

https://cmake.org/cmake/help/latest/variable/PROJECT-NAME_SOURCE_DIR.html?highlight=project%20dir

[DraTeots]

I'm surprised there is a need for a new EICRECON_SOURCE_DIR. CMake provides so many *_DIR variables already that I doubt this is necessary...

Usually they point to root or current project. Not specific if EICRECON is in the middle of the chain (it is not root anymore). There is a named _SOURCE_DIR but I'll explain further why introducing a variable is better.

https://cmake.org/cmake/help/latest/variable/PROJECT-NAME_SOURCE_DIR.html?highlight=project%20dir

Indeed it is better to be renamed to EICRECON_ROOT_DIR or EICRECON_HOME_DIR (As things named in jana). From the user point or usage points (like specific plugin CMake) it is better to rely on a special variable rather than on some project name. First it shows the intent better (and the intent to get ROOT of EICRecon dir rather than some specific project EICRecon). And the same goes to reliability - it is better to rely on a variable designed for it rather than on a fact where the project is located. Finally there are 2 eicrecon projects. One is the root project and the secon is executable eicrecon. So I think introducing EICRECON_ROOT_DIR is good from reliability, readability and unambiguousness points.

[wdconinc]

Which project (besides your clion settings) is this expected to affect?

[DraTeots]

Any project or setup that does add_subdirectory(EICRecon) will fail.

[wdconinc]

Since EICrecon will be reworked, let's just approve this and free up the queue for other issues and pull requests.