ARC4 Compilation modifications and instructions

[cemac-ccs]

• Arc4 fcm-make configuration added

• Changes brought across from trunk to allow compilation with gcc version >=7

• Fixes identified by @MarkUoLeeds in issue #11 (point 1) implemented

• Arc based compilation script added to allow stable compilation with single terminal command

• Example sge script added for running model on arc4

• Tested compilation and running with straka experiment configuration using all installed versions of gcc ind intel compilers on arc4, and using all MPI libraries. Determined that, at least in testing, monc will compile and run on arc4 with gcc versions 4.8.5 (native), 6.3.0 and 8.3.0, and with mvapich2, openmpi and intelmpi.

• Compilation instructions added in README

• Note about compiler compatability added to README

• Note about MPI libraries added to README

[leifdenby]

@cemac-ccs this is fantastic! Well done! I'll give this a proper read through and we can get it merged in

[leifdenby]

Ok. I think we should just put this into master. Does that sound ok to you @stevenleeds?

[sjboeing]

Hi all. Apologies for not getting back on this before, but ye, it all looks good and I am happy for you to merge it Leif. Chris, thank you for putting effort into this!

[leifdenby]

Thanks @sjboeing and @cemac-ccs! I just merged this in :tada: :rocket: :smile: