enhance comments since PLUGIN needs good documentation for others

/*

    This implements loading and saving a BOARD, behind the PLUGIN interface.

    The definitions:

    *) a Board Internal Unit (BIU) is a unit of length that is used only internally

       to PCBNEW, and is nanometers when this work is done, but deci-mils until done.

    The philosophies:

    *) a BIU is a unit of length and is nanometers when this work is done, but deci-mils until done.

    *) BIUs should be typed as such to distinguish them from ints.  This is mostly

       for human readability, and having the type nearby in the source supports this readability.

    *) Do not assume that BIUs will always be int, doing a sscanf() into a BIU

       does not make sense in case the size of the BUI changes.

       does not make sense in case the size of the BIU changes.

    *) variables are put onto the stack in an automatic, even when it might look

       more efficient to do otherwise.  This is so we can seem them with a debugger.

    *) Global variables should not be touched from within a PLUGIN, since it will eventually

       be in a DLL/DSO.  This includes window information too.  The PLUGIN API knows

       nothing of wxFrame or globals.

       nothing of wxFrame or globals and all error reporting must be done by throwing

       an exception.

    *) No wxWindowing calls are made in here, since the UI resides higher up than in here,

       and is going to process a bucket of detailed information thrown from down here

       in the form of an exception if an error happens.

       Simply avoiding strtok() more often than the old code washes out performance losses.

       Remember strncmp() will bail as soon as a mismatch happens, not going all the way

       to end of string unless a full match.

    *) angles are in the process of migrating to doubles, and 'int' if used, is only shortterm.

    *) angles are in the process of migrating to doubles, and 'int' if used, is

       only shortterm, and along with this a change, and transition from from

       "tenths of degrees" to simply "degrees" in the double (which has no problem

       representing any portion of a degree).

*/