SPI model fixes

[anth-volk]

Fixes #910
Fixes #901
Fixes #899
Fixes #911
Fixes #896

PR makes various changes to the UK country package to calibrate our output to the SPI 2020/21 dataset. It also ensures that we properly calculate Scottish income taxes.

[anth-volk]

Income tax error as of commit 36489be

<=10      74.696095
<1000     16.799956
<100       4.321959
>=1000     4.181990

[anth-volk]

Income tax error as of commit 1f9766c

<=10      76.810711
<1000     15.208277
<100       4.141281
>=1000     3.839731

[anth-volk]

As of commit 3944d41:

<=10      77.089142
<1000     14.974576
<100       4.146056
>=1000     3.790227

[anth-volk]

As of commit 3badd07:

<=10      77.282008
<1000     14.909617
<100       4.069412
>=1000     3.738964

[anth-volk]

Further testing seems to demonstrate that the SPI doesn't properly apply the Property Allowance. Without the allowance, as of our last commit, we had the following error rates:

<=10      77.353626
<1000     14.793143
<100       4.063255
>=1000     3.789976

If I set the Property Allowance to 0 within the parameters, the output jumps to:

<=10      83.907681
<1000      8.406449
<100       4.066396
>=1000     3.619475

This is supported by certain particular records, such as 630113, which has no employment income, a dividend income of £72,000, and a property income of £28,200, so we'd expect an adjusted net income of £99,200 (property allowance is calculated before all other allowances, and only on property), but SPI reports a TAXINC value of £100,200.

[anth-volk]

Note, too, that I tried various other approaches to fixing the property allowance issue, including changing where the property allowance was calculated, updating it and the dividend allowance parameter value, and checking to ensure that tax is assessed correctly on both types of income, but none of those were successful.

[anth-volk]

As of commit 1d6dc20:

<=10      84.148543
<1000      8.200767
<100       4.079463
>=1000     3.571227

[anth-volk]

As of commit 3660a06:

<=10      84.196289
<1000      8.156037
<100       4.081222
>=1000     3.566452

[anth-volk]

As of commit b18fffe:

<=10      85.897908
<1000      6.371240
<100       4.164777
>=1000     3.566075

[anth-volk]

As of commit a07861e:

<=10      87.994431
<1000      5.434049
<100       3.342928
>=1000     3.228591

[anth-volk]

As of commit fa70a97:

<=10      88.222981
<1000      5.449881
<100       3.340792
>=1000     2.986346

[anth-volk]

As of commit 47bb39c:

<=10      88.232530
<1000      5.448373
<100       3.335515
>=1000     2.983582

[anth-volk]

As of commit 2df0f27:

<=10      88.402905
<1000      5.363311
<100       3.297822
>=1000     2.935962

[anth-volk]

As of commit 4592b4b:

<=10      88.402905
<1000      5.363562
<100       3.297822
>=1000     2.935711

[anth-volk]

As of commit 255de32:

<=10      91.265491
<1000      5.586080
>=1000     1.999022
<100       1.149407

[anth-volk]

Final outcome:

<=10      91.265491
<1000      5.586080
>=1000     1.999022
<100       1.149407