DateTime::setTimestamp should accept int|float $timestamp 

### Description

much like the constructor accepts
```
new DateTime("@0.123456"); // 1970-01-01 00:00:00.123456
new DateTime("@".microtime(true));
```
setTimestamp should also accept 
```
$dt->setTimestamp(0.123456);
$dt->setTimestamp(microtime(true));
```
