Add PRN-ranging ambiguity

Closes #90 (closed)

• Add parameter prn_ambiguity to Instrument.__init__() to specify the cycle length -- default value set to None to keep the current behavior unchanged
• Add intermediary quantity mprs_unambiguous for low-frequency clock-sampled MPRs without PRN ambiguity
• New final quantity mprs for low-frequency clock-sampled MPRs with PRN ambiguity (if prn_ambiguity is not None)
• Add unit tests for PRN ambiguity