Use complexes directly in `abstract_sequence`

[xtrojak]

Allow usage of complexes directly in abstract_sequence - currently only aliases in place of complex are allowed

[xtrojak]

Example what is allowed now:

#! rules
S{i}:A():A2:cell => ...

#! complexes
A2 = A().A()

What is not allow but we want to allow it:

#! rules
S{i}:A():A().A():cell => ...

The problem should be located somewhere here: https://github.com/sybila/eBCSgen/blob/734fdfb1bd6a2ffc4fa4e049a400f52b772ee095/eBCSgen/Parsing/ParseBCSL.py#L141 we allow cmplx_name and VAR, but a complex directly.

[xtrojak]

Fixed in #92.