Improved functions that check stability of network and check for existence of SHC cycle

Added second input to shc_lv_iscycle(). It now defaults to requiring that the Rho matrix be non-negative. Options added for negative and positive-non-zero cases.

Changed some of the string inputs to shc_isstable() to clean it up and make it more capable.

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