Improvements to symbolic capabilities of functions

Three new utility functions in the private directory: relopsym(), isfinitesym(), and isinfsym(). These are now used across many functions (isinfsym() isn't used yet). Some functions that did not support symbolic connection matrices now do so: shc_lv_iscycle(), shc_lv_isstable(), and shc_lv_stability(). To be fully useful this will require that network parameter variable include assumptions. The next commit will include a new function to to handle this automatically.

Two new options were added to shc_lv_isstable(), 'prod' and 'margin'. See the help for details.

shc_lv_symequilibria() now uses shc_lv_ode().

Signed-off-by: Andrew D. Horchler <adh9@case.edu>