Add thick traget X-ray baremsstrahlung model

Author: Shane Maloney

requirements.txt

@@ -0,0 +1,73 @@
+"""
+Class and code to easily switch between astropy constants and values taken directly from SSW for
+comparision.
+"""
+
+from astropy import constants as const
+
+CONSTANTS = {
+    'idl': {
+            'mc2': 510.98,  # electron rest mass keV
+            'clight': 2.9979e10,  # cm s^-1
+            'au': 1.496e13,  # cm
+            'r0': 2.8179e-13,  # classical electron radius cm
+            'alpha': 7.29735308e-3,  # alpha
+            'twoar02': 1.15893512e-27  # 2 * alpha * r0^2
+        },
+
+    'astropy': {
+            'mc2': (const.m_e * const.c**2).to('keV').value,
+            'clight': const.c.cgs.value,
+            'au': const.au.cgs.value,
+            'r0': (const.a0*const.alpha**2).cgs.value,
+            'alpha': const.alpha.cgs.value,
+            'twoar02': 2.0 * const.alpha.cgs.value * (const.a0*const.alpha**2).cgs.value**2
+        }
+    }
+
+
+class Singleton(type):
+    """
+    A singleton metaclass
+    """
+    _instances = {}
+
+    def __call__(cls, *args, **kwargs):
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
+        return cls._instances[cls]
+
+
+class Constants(metaclass=Singleton):
+    """
+    Centralised constant representation
+    """
+    def __init__(self, ref='idl'):
+        """
+
+        Parameters
+        ----------
+        ref
+        """
+        self.ref = ref
+
+    def get_constant(self, name):
+        """
+        Return the value of the constant.
+        Parameters
+        ----------
+        name : str
+            Name of constant
+
+        Returns
+        -------
+        float
+            Value of constant
+        """
+        if self.ref not in ['astropy', 'idl']:
+            raise ValueError(f'Valid values for ref are astropy or idl not {self.ref}.')
+
+        if name not in CONSTANTS[self.ref].keys():
+            raise ValueError(f'Valid names are {CONSTANTS[self.ref].keys()} or idl not {name}.')
+
+        return CONSTANTS[self.ref][name]