Simplify PDB reader and add docs #1220

Job	Run time
Build MDANSE and run tests on ubuntu-latest	16m 3s
Build MDANSE and run tests on ubuntu-latest	13m 41s
Build MDANSE and run tests on ubuntu-latest	13m 8s
Build MDANSE and run tests on ubuntu-latest	14m 40s
Build MDANSE and run tests on ubuntu-latest	14m 42s
Build MDANSE and run tests on windows-latest	17m 39s
Build MDANSE and run tests on windows-latest	17m 46s
Build MDANSE and run tests on windows-latest	17m 23s
Build MDANSE and run tests on windows-latest	17m 56s
Build MDANSE and run tests on windows-latest	18m 32s
Build MDANSE and run tests on macOS-latest	9m 13s
Build MDANSE and run tests on macOS-latest	7m 30s
Build MDANSE and run tests on macOS-latest	7m 39s
Build MDANSE and run tests on macOS-latest	8m 10s
Build MDANSE and run tests on macOS-latest	7m 46s
	3h 21m 48s

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