added CURRENCY_ATOM to express minimum indivisible unit

also moved CURRENCY_* into feerate.h file to work around MSVC bug

src/policy/feerate.cpp

@@ -7,8 +7,6 @@
 
 #include <tinyformat.h>
 
-const std::string CURRENCY_UNIT = "BTC";
-
 CFeeRate::CFeeRate(const CAmount& nFeePaid, size_t nBytes_)
 {
     assert(nBytes_ <= uint64_t(std::numeric_limits<int64_t>::max()));

src/policy/feerate.h

@@ -11,7 +11,8 @@
 
 #include <string>
 
-extern const std::string CURRENCY_UNIT;
+const std::string CURRENCY_UNIT = "BTC"; // One formatted unit
+const std::string CURRENCY_ATOM = "sat"; // One indivisible minimum value unit
 
 /**
  * Fee rate in satoshis per kilobyte: CAmount / kB