Specify PDB file format requirements in help tour

[touwwouter]

The minimal requirements of the PDB file should be listed in the help tour. This should prevent users from uploading PDB files in a format mkdssp cannot handle and should in combination with #63 and #64 improve feedback.

Examples:

• All backbone atoms should be present (O is not used for secondary structure definition, but is necessary for calculation and output of the C=O-C=O torsion angle).
• If a SOURCE record is present, it should not be empty (xssp fails on empty SOURCE records hssp#53).
• B-factors (or other values in the B-factor column) should have meaningful values if present (mkdssp: handle missing B-factors hssp#54).
• Occupancy should be present (Support coordinate-only PDB files hssp#52).

[jonblack]

@touwwouter
Are the bullet-points above the exact text you want in the help tour?

[touwwouter]

Almost. Let's specify all text when we have decided what the format of the PDB file should be (cf. other issues in xssp and xssp-rest).

[touwwouter]

The requirements have been specified now. Start a new issue if they need to be changed.