Problems with rework of `TwoLevelTree`

[JulStraus]

The current version is unfortunately no longer compatible with TimeStruct for TwoLevelTree types. The main problem is related to renaming of internal functions, most likely from PR #82,

The following error is given:

ERROR: UndefVarError: `nchildren` not defined in `TimeStruct`

I guess the problem could be solved by changing l. 335

from

bm = bm + (0, h * max(1, TimeStruct.nchildren(node, ts)))

to

bm = bm + (0, h * max(1, TimeStruct.n_children(node, ts)))

[trulsf]

Fixed in #4