Fix panic when eliding invalid constant cases

[benpaxton-hf]

+, >, <, <=, and >= require typechecking of both operands
simultaneously; constant-operand eliding did not perform this check,
causing a panic.

In the below examples, conditional.go is a test program I wrote when investigating this library; it is essentially a minimal REPL for govaluate.

Before:

$ go run conditional.go 
> 1 > true
panic: interface conversion: interface is bool, not float64

goroutine 1 [running]:
panic(0x4e0180, 0xc42001a0c0)
	/usr/local/go/src/runtime/panic.go:500 +0x1a1
github.com/Knetic/govaluate.gtStage(0x4d5ac0, 0xc42000a188, 0x4d5900, 0xc42000a198, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/repos/go/src/github.com/Knetic/govaluate/evaluationStage.go:103 +0xb7
github.com/Knetic/govaluate.elideStage(0xc4200a00f0, 0xc4200a00a0)
	/repos/go/src/github.com/Knetic/govaluate/stagePlanner.go:662 +0x254
[...]
exit status 2

After:

$ go run conditional.go 
> 1 > true
Evaluate: Value '1' cannot be used with the comparator '>', it is not a number
> 

The error message is still incorrect, but it's better than a panic. Likely the error message should be something along the lines of "Values '1' and 'true' cannot be used with the comparator '>'".

[Knetic]

Looks great to me!

Thanks for using the library, and taking the time to put together a PR!