Make import keyword change working directory during external compilation

[WoodchuckXL]

Fix for #277.
Changes the global CWD to the current file path during compilation of imported files to allow for them to import files with local paths. The CWD is restored after compilation of the imported file is finished.
Added test for this behavior.

[softmattertheory]

This is a very useful fix; thanks for the great contribution! Here are a few suggestions to better integrate with existing morpho functionality:

1. There's a macro MORPHO_DIRSEPARATOR that gives the folder separator that might be better to use than hard coding '/'.

2. Changing the current directory is platform dependent, so it may be useful to use the existing functions in platform.c to help:

bool platform_setcurrentdirectory(const char *path);
bool platform_getcurrentdirectory(char *buffer, size_t size);