Clean compilation

[EmilyBourne]

Overhaul the compilation method. A class Compiler is now used to handle all compilation steps. A class CompileObj is used to store all information required to compile a given file. Default compile configurations for basic compilers (gcc, gfortran, icc, ifort) are saved in json files in pyccel/compilers these are determined thanks to the sysconfig module. Fixes #541.

After this PR, tests run much faster. The times below show the last 3 tests run on the master and ebourne_compiling branches, followed by the average times
Master branch

Linux (python3.6)	MacOs	Windows
19m37	16m54	28m34
18m46	21m13	24m28
21m13	19m53	23m54
19m52	19m20	25m39

ebourne_compiling

Linux (python3.6)	MacOs	Windows
13m13	17m23	15m17
11m59	16m17	15m25
15m30	17m52	18m01
13m34	17m11	16m11

Supported compilers:
The following compilers have been tested:

• GNU : gcc/gfortran
• intel : icc/ifort (tested by @EmilyBourne )
• PGI : pgcc/pgfortran (tested by @saidctb )
• nvidia : nvc/nvfort (tested by @rakati )

Commit Summary

• Create CompileObj to hold all information required to compile a given file
• Create Compiler to hold all information about a given compiler
• Remove cwrapper.py. The setup_X.py file is no longer required
• Use Compiler and CompileObj to simplify pipeline.py and python_wrapper.py
• Allow multiple accelerators
• Make mpi an accelerator instead of specifying the mpi_compiler
• Specify the compiler family (e.g. GNU) instead of the compiler name
• Add a wrapper_flags argument to separate the flags for the file (fortran) from the flags for the wrapper (c)
• When compiling a program, compile everything as modules then link separately. This procedure is closer to the procedure used for shared libraries
• Rename pyc_math to pyc_math_c and pyc_math_f90. This allows us to differentiate between the .o files created from the source files which means we don't need to systematically recompile the library
• Only overwrite stdlib files if they are out of date
• Only recompile stdlib files if the source files have been updated
• Add pyccel.compilers.generate_default to generate the default json files
• Lock file during pickling. Fixes Pickling and unpickling should use flock #917
• Add a file describing how to use different compilers

[EmilyBourne]

It seems either impossible or inadvisable to generate the json during the installation. See here