Fortran Warning Fixes, master branch (2022.03.22.)

Made sure that all DO-loops would end in a CONTINUE statement, and that nested DO-loops would not share the same CONTINUE statement. Which apparently are both removed as a possibility from Fortran 2018.

Note that there are a number of places in the code that used to look like the following:

       DO 10 I=1,10
         DO 10 J=1,10
           ...
           IF (<SOMETHING>) GOTO 10
           ...
 10    CONTINUE

I have no idea how this should be fixed. 😦 Should it be

       DO 10 I=1,10
         DO 20 J=1,10
           ...
           IF (<SOMETHING>) GOTO 10
           ...
 20      CONTINUE
 10    CONTINUE

or

       DO 10 I=1,10
         DO 20 J=1,10
           ...
           IF (<SOMETHING>) GOTO 20
           ...
 20      CONTINUE
 10    CONTINUE

??? I went with the latter one in this MR. But somebody more versed in Fortran should really speak up... 🤔 @ssnyder, are you sure about this part? (After some tests, as shown in the MR's discussion, the latter one turned out to be the correct solution.)

This is meant as a replacement for !51454 (closed), fixing/addressing AGENE-2140 and ATLINFR-4464.

Pinging @fwinkl as well, just for good measure. 😛