basic.select method can be faster when element is diferent from 'atom' and syntax='MolSysMT'

[dprada]

When a selection is done with element different from 'atom' over a molecular system with a foreign form, multiple conversions are executed and can be avoided. For instance:

• The native MolSysMT selector needs a conversion to 'molsysmt.Topology'.
• The atom_indices to element indices filter needs another conversion to 'molsysmt.Topology'.
• If the argument mask is used, another conversion to 'molsysmt.Topology' is required.

The process can be optimized with a single conversion in case syntax='MolSysMT'.