Bugfix: fix 2 bugs about solvent and cache

[shaoqx]

Fix 2 bugs:

1. bug discovered by @SwordJack about MDCRD parser
2. a potential caused by cache var

[SwordJack]

Hi Shao,

I've added round brackets to calls to the stru.structures method in enzy_htp.analysis.spi_metric. The test will raise following TypeError unless the round brackets are added.

>       for ss in stru.structures:
E       TypeError: 'method' object is not iterable

../../enzy_htp/analysis/spi.py:80: TypeError
================================== short test summary info ==================================
FAILED test_spi.py::test_spi_consistent_with_old_enzyhtp - TypeError: 'method' object is not iterable
=============================== 1 failed in 133.53s (0:02:13) ===============================

After the modification, the test is now able to be passed.

Best, Zhong.