Improving installation and user guide

[x] How about doing cp libyt.so* ../lib/ automatically at the end of Make?
[x] Use make make install
[x] Clarify how to determine PYTHON_PATH, PYTHON_VERSION, NUMPY_PATH.
[ ] Do you think it would be helpful to validate the minimum required versions of yt and python? (Move to #81 )
[x] Single out libyt frontend to data-exp-lab/yt_libyt

[x] Remove / from the beginning of, for example, /libyt/src, /libyt/lib, and /libyt/include in the installation guide.
[x] Add cd yt before pip install . here.
[x] For %libyt load here, change to "All NEW functionS detected ..."
[x] Have you mentioned the usage of LIBYT_STOP in the libyt user guide?
[x] #80
[x] I don't quite understand the following comment here (and fix the typo was --> were). Try to improve it.

When calling yt_run_InteractiveMode, libyt will run all the functions that was set to run using %libyt run, but had not been run by yt_run_Function or yt_run_FunctionArguments yet.

yt-project / libyt