Tweak POSCAR / XDATCAR to accommodate malformed files

[esoteric-ephemera]

Related to this matsci.org issue: sometimes the XDATCAR can be malformed because fortran uses fixed format floats when printing. In those cases, there's no space between coordinates:

Direct configuration= 2
-0.63265286-0.11227753 -0.15402785
-0.12414874 -0.01213420 -0.28106824
...

In some cases, this issue is reparable (a negative sign separates coordinates). This PR implements a fix when it is reparable and adds a few Trajectory-like convenience features to Xdatcar

[shyuep]

Thanks.