Merge of to_nest() and nest_install()

[clinssen]

to_nest() requires the user to specify the directory where generated code will be written into. The same directory needs to be specified as a parameter to install_nest(). If these functions are merged, and the user does not particularly care where the model is generated, there should be no need to specify a target path.

The target location should default to a place where NEST can find the generated .so file without additional specification of LD_LIBRARY_PATH. Possibly, nest-config should be interrogated for the requisite path.

[Silmathoron]

See also #480

[clinssen]

One potential solution could be:

• rename to_nest(...) to generate_target_code(target="nest", ...)
• rename install_nest(...) to build_target_code(target="nest", ...)
• add a new function generate_build_target_code(target="nest", ...) that simply calls the first two in order.

Side points:

• pynestml/utils/model_installer.py is NEST-specific and should be moved to pynestml/codegeneration (or a subdirectory in there)
• same for NEST-specific functions in pynestml/frontend/pynestml_frontend.py

[tfardet]

Are there plans to make NESTML not NEST specific? Given the name this seems like a strange idea ^^"
If not, then I would rather try to avoid breaking all user codes...
I'd be fine with adding a new function combining both previous functions without renaming them, though