Implement support for `xEC` bodies

[amereghe]

Both infinite bodies (i.e. XEC, YEC and ZEC), but also the finite version (i.e. REC).
This implies:

• input parsing / geometry echoing. This inlcudes implementing parsing multiple input lines and echoing on multiple lines;
• converting to and from infinite bodies (for geometry manipulations);
• adding a couple of arrays in the definition of the Body class;