Allow variable values in ocean_month.py

[forsyth2]

In zppy/templates/ocean_month.py, Lines 32-35, 10 is hard-coded:

for y in range(start_yr, end_yr, 10):

    year1 = y
    year2 = y + 10 - 1

This was causing issues addressing #128 because the tests were using fewer than 10 years.

A related issue, on line 29: tunits = "days since 0001-01-01 00:00:00", which I believe is only true if we're running from year 0001 and not say 1850.

[xylar]

Another issue is that these constants should come from mpas_tools.cime.constants instead of being hard-coded:

zppy/zppy/templates/ocean_month.py

Lines 20 to 25 in 9793c65

	# Ocean constants
	# specific heat [J/(kg*degC)]
	cp = 3.996e3
	# [kg/m3]
	rho = 1026.0
	fac = rho * cp

from mpas_tools.cime.constants import constants
...
# specific heat [J/(kg*degC)]
cp = constants['SHR_CONST_CPSW']
# [kg/m3]
rho = constants['SHR_CONST_RHOSW']
fac = rho * cp

This will require adding a new dependency on mpas_tools.

[xylar]

A related issue, on line 29: tunits = "days since 0001-01-01 00:00:00", which I believe is only true if we're running from year 0001 and not say 1850.

I don't think this is a problem. It might be more aesthetic to have:

tunits = f"days since {start_yr:d04}-01-01 00:00:00"

but your later calls to date2num( take tunits as an argument so they will work as expected regardless of the reference year.

[xylar]

year2 = y + 10 - 1

Presumably this should be the following?

year2 = min(y + 10 -1, end_yr)