[Feature]: GC Clamp

[sepandhaghighi]

Describe the feature you want to add

GC clamp is the presence of a guanine (G) or cytosine (C) base in the last 5 bases of a primer.

Describe your proposed solution

>>> from opr import Primer
>>> p1 = Primer("ATCG")
>>> print(p1.gc_clamp)
0
>>> p2 = Primer("ATCGG")
>>> print(p2.gc_clamp)
3

ℹ️ The GC clamp is only applicable to primers that are longer than 4 nucleotides.

Describe alternatives you've considered, if relevant

No response

Additional context

Test case: https://www.oligoevaluator.com/ShowToolServlet?TYPE=ANALYSIS
More info: https://toptipbio.com/gc-clamp-pcr/