Add /MAX=... as a rename of /SIZE=... in SET MEMORY

[karlmsmith]

(From GFDL 2019 visit)
Since /SIZE=... in SET MEMORY is now a maximum allocation limit and not an actual allocation, rename this option to /MAX=... ; however, keep the /SIZE=... for backwards compatibility.
Update documentation to use SET MEMORY /MAX=...

[AnsleyManke]

A particularly useful side effect of this will be to make the SHOW MEMORY outputs more meaningful, saying MAX= instead of /SIZE=

yes? set mem/max=.5G
yes? show  memory
    SET MEMORY/MAX: 500 Mb   (62.5 megawords)
    Peak demand: 0 Kb
    Current cache: 0 Kb

yes? set mem/max=1G
yes? show  memory
    SET MEMORY/MAX: 1 Gb   (125 megawords)
    Peak demand: 0 Kb
    Current cache: 0 Kb

! do some work, check memory diagnostics

...

yes? use  9201F958F9D6A4514E38980EADB9291F_netcdf.nc
yes? set mem/siz=0.5G
yes? let var = xsequence(ship_speed)
yes? list var[i=@max]
             VARIABLE : XSEQUENCE(SHIP_SPEED)
             DATA SET : SOCAT v2019 Data Collection
             FILENAME : 9201F958F9D6A4514E38980EADB9291F_netcdf.nc
             FILEPATH : /home/users/ansley/tomcat8/webapps/las/output/
             X        : 0.5 to 121777.5 (maximum)
          24.30
yes? show mem/diag
    No split/gather occurred in the last evaluation
    Total table slots: 500
    Free table slots: 497
    Un-cached variables: 0
    SET MEMORY/MAX: 500 Mb   (62.5 megawords)
    Peak demand: 1.95 Mb
    Current cache: 1.95 Mb

[AnsleyManke]

This is done, and checked in on the Ferret trunk. /MAX appears in error and info messages.