crosslocmodel cannot read file with single cross section

[veenstrajelmer]

Describe the bug
When providing a crosslocfile with a single cross section, hydrolib-core cannot read this since it is not a list. Related to #678.

To Reproduce

from hydrolib.core.dflowfm import CrossLocModel
crsl_file = "crsloc.ini"
crsl_str = """
[General]
fileVersion = 1.01
fileType    = crossLoc

[CrossSection]
id           = 99
branchId     = 9
chainage     = 403.089709
shift        = 0.0
definitionId = 99
"""
with open(crsl_file, 'w') as f:
    f.write(crsl_str)

crossloc = CrossLocModel(crsl_file)

This raises

ValidationError: 1 validation error for CrossLocModel
crsloc.ini -> crosssection
  value is not a valid list (type=type_error.list)

Expected behavior
Also files with a single cross section should be properly read.

Version info (please complete the following information):

• OS: Windows
• Version 0.7.0