Fix go vet complaints

[anzellai]

Fix a bunch of Go vet complains, most of them are just incorrect or missing fmt arguments.
Here is the list of Before/After:

Before

anzellai/golearn - [fix/go-vet-complaints] » go vet ./...

base/categorical.go:13: struct field values has json tag but is not exported
base/conversion.go:12: result of fmt.Errorf call not used
base/filtered.go:160: arg as for printf verb %s of wrong type: base.AttributeSpec
base/filtered.go:180: arg a for printf verb %s of wrong type: base.AttributeSpec
base/fixed.go:72: wrong number of args for format in Sprintf call: 1 needed but 2 args
base/serialize.go:135: struct field tag json:type not compatible with reflect.StructTag.Get: bad syntax for struct tag value
base/serialize.go:136: struct field tag json:name not compatible with reflect.StructTag.Get: bad syntax for struct tag value
base/serialize.go:137: struct field tag json:attr not compatible with reflect.StructTag.Get: bad syntax for struct tag value
exit status 1
clustering/clustering.go:69: arg clusterIdMap for printf verb %s of wrong type: map[int]int
clustering/clustering.go:100: arg ref[cNew] for printf verb %s of wrong type: []int
exit status 1
filters/binning.go:34: arg b.attrs for printf verb %d of wrong type: map[invalid type]bool
filters/chimerge_freq.go:13: arg t.Frequency for printf verb %s of wrong type: map[string]int
exit status 1
meta/bagging_test.go:17: possible formatting directive in Fatal call
meta/bagging_test.go:42: possible formatting directive in Fatal call
exit status 1

After

anzellai/golearn - [fix/go-vet-complaints●] » go vet ./...
anzellai/golearn - [fix/go-vet-complaints●] »