litescope/core: set default csr_csv to csr_csv="analyzer.csv".

Simplify creating basic analyzer:

from litescope import LiteScopeAnalyzer
analyzer_signals = [...]
self.submodules.analyzer = LiteScopeAnalyzer(analyzer_signals, depth=512)
self.add_csr("analyzer")

litescope/core.py

@@ -226,7 +226,7 @@ class _Storage(Module, AutoCSR):
 
 
 class LiteScopeAnalyzer(Module, AutoCSR):
-    def __init__(self, groups, depth, clock_domain="sys", trigger_depth=16, csr_csv=None):
+    def __init__(self, groups, depth, clock_domain="sys", trigger_depth=16, csr_csv="analyzer.csv"):
         self.groups = groups = self.format_groups(groups)
         self.depth  = depth