rename (or drop?) SIRF_SRC_PATH

[KrisThielemans]

we have 2 variables SIRF_PATH (set to /home/sirfuser/devel/buildVM/sources/SIRF/) and SIRF_SRC_PATH (set to /home/sirfuser/devel). The first is pointing to the source, and its name is determined by code in SIRF. The second is a VM variable only. It was introduced to allow us to put things elsewhere than ~/devel.

It's used in 2 places essentially (aside from internal usage in UPDATE.sh):

• update_VM_to_full_STIR.sh to know where to install STIR-exercises (but it could get this from SIRF_PATH/../../..
• UPDATE.sh checks the existence of this variable and if does puts everything there. At the end, it writes it in ~/.sirfrc.

I suggest therefore

• to remove it from update_VM_to_full_STIR.sh ,
• give UPDATE.sh an option to specify it, which defaults to SIRF_SRC_PATH if it exists or ~/devel otherwise.
• No longer put it in ~/.sirfrc