Allow symbol checks to be disabled

[arporter]

The new symbol table functionality is somewhat immature (it cannot cope with use association) and only works if the supplied source is valid Fortran, even if any pre-processor directives are ignored. These limitations mean that it can be tripped up in certain circumstances. While support for use association is planned (#201 and #349), there's no plan to handle CPP directives. Therefore we will allow the checking performed by the symbol table to be disabled at run time.

I'm not sure whether this will be best done by completely disabling the symbol table functionality or by only disabling the checks that a symbol does not already exist.

[arporter]

@rupertford has suggested that since the symbol-table support is essentially still beta, we should default to disabling these checks.