merge_consecutive_drifts() can prevent aperture misalignment

[roman-martin]

Example assumes #38 to be fixed

import cpymad.madx
import pysixtrack

# code from help(pysixtrack.Line.apply_madx_errors) with minimal madx sequence
madx = cpymad.madx.Madx()
madx.input('''
    !----EXAMPLE----------------------------------------------------------------
    MQ1: Quadrupole, K1:=KQ1, L=1.0;
    MQ2: Quadrupole, K1:=KQ2, L=1.0, apertype=circle, aperture=0.2;

    KQ1 = 0.02;
    KQ2 = -0.02;

    testseq: SEQUENCE, l = 20.0;
        MQ1, at = 5;
        MQ2, at=18;
    ENDSEQUENCE;

    !---the usual stuff
    BEAM, PARTICLE=PROTON, ENERGY=7000.0, EXN=2.2e-6, EYN=2.2e-6;
    USE, SEQUENCE=testseq;


    Select, flag=makethin, pattern="MQ2", slice=2;
    makethin, sequence=testseq, style=collim;

    use, sequence=testseq;

    !---misalign collimator
    select, flag = error, clear;
    select, flag = error, pattern = "MQ2";
    ealign, dx=-0.04;

    !---/EXAMPLE----------------------------------------------------------------
''')
seq = madx.sequence.testseq
madx.command.esave(file='lattice_errors.err')
madx.command.readtable(file='lattice_errors.err', table="errors")
errors = madx.table.errors
pysixtrack_line = pysixtrack.Line.from_madx_sequence(seq,install_apertures=True)
pysixtrack_line.merge_consecutive_drifts(inplace=True);
pysixtrack_line.apply_madx_errors(errors)

print('\n')
print('Line content:')
for name,element in zip(pysixtrack_line.element_names,pysixtrack_line.elements):
    print("{0:30} : {1}".format(name,element))

The example creates a sliced quad that has a marker with an aperture in the middle. If merge_consecutive_drifts() is called before apply_madx_errors() the centre aperture is not misaligned because the corresponding zero-length drift is gone.

Solution: either search for aperture markers if an element is not found, or make sure that zero length drifts are only removed after applying errors.

[aoeftiger]

I believe we should only allow the order where mad-x errors are applied first.

pysixtrack_line.apply_madx_errors(errors)
pysixtrack_line.merge_consecutive_drifts(inplace=True);