Merge branch 'taufiqhassan/eamxx/mamxx-time-interp-fix' (PR #7381)

EAMxx: Fixed temporal interpolation for external forcings

Previously, prescribed vs EAMxx elevated emissions (bc_a4 for example) showed significantly large discrepancies:

   months     prescribed_emis          EAMxx    rel_diff (%)
0     jan  50.720847076634016        41.87359    21.128494
1     feb  33.052801575716245       33.808193    -2.234345
2     mar  34.564076987260805        34.10844     1.335847
3     apr    33.6571496550456        34.85354    -3.432618
4     may   36.04996553155214        46.04442   -21.706112
5     jun   56.01655671895189        71.36652   -21.508638
6     jul    86.6789942679998        98.19908   -11.731359
7     aug  109.69631225550144      105.578384     3.900351
8     sep   101.4825383463081         72.2314    40.496431
9     oct   43.06931282750091        37.60891    14.518909
10    nov   32.16846281675161       40.085526   -19.750428
11    dec  47.985367140309215       49.352615    -2.770366
As opposed to EAM-

   months     prescribed_emis            EAM        rel_diff (%)
0     jan   65.53070785855884    58.4741778369735    12.067771
1     feb  32.950843360402935   36.67505759330244   -10.154624
2     mar  33.296972174272504   32.59856770545893     2.142439
3     apr  28.873338098498838    31.1443850952515    -7.291995
4     may  41.523370000927116   41.45759393878491     0.158659
5     jun   49.88711151401323  54.040493434934405    -7.685685
6     jul   85.16360864281151    82.8069247826244     2.845999
7     aug   94.06291468483843   90.19021139784672     4.293929
8     sep   74.23516764683626   70.60245248538006      5.14531
9     oct   33.96152809508068  37.868317940064216   -10.316777
10    nov  31.705675644882124   33.74076635265753    -6.031549
11    dec   44.28931694279417   46.16966256252293    -4.072687
With this fix, prescribed vs EAMxx show similar behaviour as EAM
(Using identical emissions in the reference EAM run above)

   months     prescribed_emis      EAMxx   rel_diff (%)
0     jan   65.53070828175305   58.45631    12.102026
1     feb    32.9508434047515  36.658974   -10.115205
2     mar   33.29697212313313  32.588203     2.174924
3     apr   28.87333817845113  31.138515    -7.274519
4     may   41.52337000418099  41.451073     0.174416
5     jun   49.88711150420784  54.034306    -7.675113
6     jul   85.16360885120517  82.792755       2.8636
7     aug   94.06291452961088   90.16316     4.325217
8     sep   74.23516786718005   70.56994     5.193754
9     oct  33.961528086365256   37.84876   -10.270431
10    nov   31.70567563396028  33.733593    -6.011566
11    dec  44.289317262530815  46.165443    -4.063919
Note:

These changes apply only to external forcings. Other tracer types (e.g., oxidants, LINOZ) still follow the existing/default method.
The current implementation is only compatible with emission datasets that follow a periodic annual cycle.

[NBFB]

* taufiqhassan/eamxx/mamxx-time-interp-fix:
  added some comments for clarity
  initial fix for ELEV emis time interpolation

Author: singhbalwinder