Make XMLParser less picky about dtd grammar file locations

[Sv3n]

Custom memspecs might not sit in the same folder as the dtd file that is delivered with DRAMPower, causing the parser to fail. Locally I've fixed this by adding an EntityResolver to the parser, which attempts to load the dtd from a path that is:

• relative to the xml file (default behavior)
• relative to the current working directory
• relative to an environmental variable (such that you can point it to the DRAMPower folder for example)

I propose to add this functionality in the official version as well, since it does no harm. Long term we might want to consider switching to a less wacky xml parser, and perhaps a grammarless structure (since we don't really rely on it anyway).