Removed ReplaceWith attribute for Qsl_Via 'M' enumeration because
AdifEnumerations.GetReplacement should return null rather than an
string.Empty for this value.
Added ModifyValues to Qsl_Rcvd_Via and Qsl_Sent_Via classes to
test if Qsl_Via enumeration value is deprecated and replace with
ReplaceWith attribute value or null as appropriate.
User manual updated to show that for Qsl_Rcvd_Via and Qsl_Sent_Via
status of 'M', the field is deleted.
This completes issue #33.