Add capability to recalibrate holding drift=0

[ljdursi]

A new argument is added to recalibrate_model, scale_drift; if false, only the two equations for scale and shift are solved, and the model drift is set to zero. As before, the var is recalculated if the scale_var option is set. Sample output from 20 reads is shown below; this is the output of scorereads without recalibration, with recalibration with drift allowed to - well - drift; and with drift head to zero (nanopolish scorereads -z).

#read                                                strand      model                                no_recalib  drift     drift=0
74f196d1-79b1-4f95-bc85-556ea3fd1103_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -4.24859    -4.23368  -4.23347
74f196d1-79b1-4f95-bc85-556ea3fd1103_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -4.4335     -4.43344  -4.43038
890d5665-065c-4989-a504-97360f394797_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -4.37701    -4.36775  -4.36776
890d5665-065c-4989-a504-97360f394797_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -4.37236    -4.36813  -4.36951
b4411ee0-5792-4ed1-9cb7-c9d5eb697666_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -4.57109    -4.55259  -4.55328
b4411ee0-5792-4ed1-9cb7-c9d5eb697666_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -4.35806    -4.3587   -4.35831
94f84f7a-e625-4b57-8531-7a63bcf730c2_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -4.1253     -4.1167   -4.11675
94f84f7a-e625-4b57-8531-7a63bcf730c2_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -3.7        -3.43947  -3.44041
cfe23418-3f4e-46ec-b56c-f54b8b5d13bf_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -3.67172    -3.67802  -3.67525
cfe23418-3f4e-46ec-b56c-f54b8b5d13bf_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -3.87516    -3.80198  -3.80236
86bceb89-437d-4ca1-ac75-6ec3304a73c2_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -3.2359     -3.23546  -3.23508
86bceb89-437d-4ca1-ac75-6ec3304a73c2_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -3.87957    -3.80286  -3.80601
1728e3d1-e257-47dd-8271-0fad5b5c7b61_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -3.35842    -3.3575   -3.37605
1728e3d1-e257-47dd-8271-0fad5b5c7b61_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -3.86848    -3.79484  -3.79543
cb0fc2d3-cf92-45d4-a62a-2bb072663d94_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -3.44349    -3.44036  -3.44074
cb0fc2d3-cf92-45d4-a62a-2bb072663d94_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -3.971      -3.88061  -3.88268
d32be6a9-a55a-422c-a8a1-35e2a36109df_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -3.51565    -3.51673  -3.52001
d32be6a9-a55a-422c-a8a1-35e2a36109df_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -4.38664    -4.29946  -4.30457
d1add053-faed-4097-bc6d-ee7817158438_Basecall_2D_2d  template    template_median68pA.model.dropmodel  -3.09503    -3.09479  -3.09496
d1add053-faed-4097-bc6d-ee7817158438_Basecall_2D_2d  complement  template_median68pA.model.dropmodel  -3.11194    -3.10402  -3.10407

[jts]

This was merged into the r9-new-model branch and subsequently into master - closing this PR.