Isrspectrum

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**Fixed:**

* Now importing ioncheck and the amu lookup into main package.

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**Added:**

* Added function to get atomic mass units for ion species.
* Added function to check if valid ion species.

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**Added:**

* Added getspecsimple function so users can avoid using data blocks.

**Changed:**

* Applied black to mathutils.py

**Fixed:**

* String naming convention for ions. Second letters are always lower in the names.

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